Workshop manual

How to use this website

Every project page on this website is designed so that the project works first time with real materials: tested measurements, pieces with templates, specific components, steps you can check and safety warnings. This manual is the part common to all projects: what is worth knowing once rather than repeating on every page.

You don't need to read it all before you start. Read this section, the one on the adult's role and the one on safety (about 25 minutes in total). Look up the rest when you need it: the cardboard techniques when you build your first vehicle, the electricity part when you make your first circuit, the section on groups if you work in a classroom.

What you will find on each project page

From the catalogue you can filter by category, age, difficulty and whether you need to buy anything. When you open a project you will see these sections, always in the same order:

Printable templates at 1:1 scale

Many projects have pieces with templates. The "Print templates at actual size (A4)" button generates one page per A4 sheet with the pieces drawn at their exact size. To get them right:

1. In the print dialog choose 100 % scale or "actual size". Turn off "Fit to page" or "Fit to printable area": it is the number one cause of pieces that don't fit (it shrinks them by 3 % to 6 %).
2. Use a ruler to measure the 50 mm red line in the header on every sheet: it must measure exactly 50 mm. If it measures 48 mm, print again after checking the scale.
3. Black and white printing is fine. Solid lines are for cutting; dashed lines, for folding; circles with a cross, holes.

How to transfer the template to cardboard is explained in Cardboard techniques. If you don't have a printer, each piece has its measurements in mm on the page and you can draw it with a ruler.

Workshop mode

The "Workshop mode (large step by step)" button shows one step per screen, in large type, designed so you can leave the tablet or laptop on the work table while you build. Move on with "Next" or the keyboard arrows. Steps that an adult must do are marked. Recommendations:

• Let the child be the one who presses "Next": that way they set their own pace and don't move on until the step's check is met.
• Read the step aloud (or let the child read it if they already read fluently) before touching anything.
• Protect the screen: a clear freezer bag or a case keeps off drops of glue and paint.

Ticking off steps, materials and finished projects

The materials and preparation lists can be ticked with a click. What you tick is saved in the browser on that device (not in an account): if you change computer or clear your browsing data, it is lost. The "Mark as done" button on each project feeds the catalogue counter and the certificate. The lists in this manual can also be ticked.

Printing

"Print page" generates a paper version without buttons or menus, about 4–8 sheets depending on the project. For a classroom session it is usually enough to print the step by step and the templates; the science for the adult is best read beforehand, on screen. This manual can also be printed in full (about 50 sheets): better to print only the sections you are going to use.

How to choose the first project

The first project has a single mission: to work and to make the child feel they did it themselves. Choose below their level, not above it. Use two criteria: age (motor skills and reading) and patience (how long they can stay focused without seeing a result).

Suggested pathways

First month. At home it is usually more realistic to do 2 sessions a week. Here is a first month of 8 sessions that gradually increases in difficulty and introduces a new technique each time:

1. Foaming volcano: measuring quantities, chemical reaction.
2. Balloon-powered car: first axles with a straw and wooden skewer.
3. Lever catapult: lever and fair test measuring distances.
4. Tube torch with a switch: first circuit.
5. Syringe digger arm: syringes, joints with split pins.
6. Three-light traffic light: polarity, resistors.
7. Rubber band car: wheels, friction, improving the prototype.
8. Solar car with belt drive: brings together everything above.

By interest.

Realistic timings

The times on the project pages are not catalogue figures: they are real build times with a child of the stated age. To plan, add up:

• Adult preparation: 10–30 min (printing, gathering materials, pre-cutting, testing components). Do it the day before if you can.
• Building: the time on the page. Multiply it by 1.5 with children aged 5–7 or if it is the first time you have done something similar.
• Drying: PVA glue 30–60 min (better overnight), hot glue 2–5 min, poster paint 20–30 min.
• Testing and improving: 10–20 min. Don't cut it short: this is where most of the learning happens.
• Tidying up: 10 min, with the children.

If the project takes more than 90 minutes, split it into two sessions: build the structure one day (and leave it to dry) and the electrical part or the decoration another.

The adult's role

The most common mistake is not about safety or technique: it is the adult ending up doing the project while the child watches. It happens without meaning to, because the adult does it faster and better, and because they want it to turn out well. The result is a nice object that the child doesn't feel is theirs and from which they have learnt nothing. Your job is different: to prepare, to look after safety, to do what the child cannot yet do safely and, above all, to ask questions.

Supporting without doing it for them

• Hands behind your back. When the child is cutting, gluing or connecting, keep your hands off the table. If you need to show something, do it on a separate scrap of cardboard (a "practice piece"), not on their piece.
• Only do yourself what is marked as adult on the page (craft knife, hot glue, drilling, fire, oven) and say so: "I'll do this cut because the craft knife is dangerous; you hold the ruler and tell me if it moves."
• Prepare things so they can do it. Pre-cutting a difficult piece, marking in pencil where a hole goes or leaving the wires already stripped is not "cheating": it is bringing the task to their level.
• Accept an imperfect result. A crooked cut that works is a good cut. If the flaw stops it working, turn it into a problem to solve together (see below); don't quietly fix it yourself.
• Sit beside them, not opposite. From the side you see the piece from the same angle and you don't give them "mirror-image" instructions.

What to say and what not to say

Open and closed questions

A closed question is answered with yes, no or one word; it is useful for checking something quickly ("Have you put the long leg on the red one?"). An open question makes them think and has no single answer; it is the one that makes them learn. Mostly use open ones, and leave at least 5 seconds of silence after asking: the awkward silence is the moment when the child is thinking.

Handling frustration and "I can't do it"

Every project has at least one moment when something doesn't work. That is normal and useful, as long as it doesn't turn into giving up. This script works in most cases:

1. Stop and name what they are feeling. "I can see you're cross because the LED won't light up. That's normal, it annoys me too." Don't play it down ("it doesn't matter") or rush to fix it.
2. A physical break of 1–2 minutes. Let them put the piece down, drink some water, get up. With children aged 5–7, this break is essential.
3. Shrink the problem. "It doesn't work" is too big. "Let's check just one thing: is the battery OK?" A small problem can be solved.
4. Let them choose the next step between two specific options: "Shall we test the battery with another LED, or check the clips first?". Choosing gives them back control.
5. If they make no progress in 10 minutes, offer more help: you do one part in front of them, explaining out loud, and they do the next.
6. If it still doesn't work, park it: "Let's stop here and look at it tomorrow with fresh eyes." Note in the notebook what you have tried. The next day, 80 % of the time, the problem is spotted in 2 minutes.

Helpful phrases: "You can't do it yet" (instead of "you can't do it"), "What have you found out doesn't work?", "Engineers try lots of times before something works".

Mistakes as data

When something fails, the project is giving you information. Treat it like a detective: a car that veers to the left is telling you one wheel rubs more than the other; an LED that doesn't light up is telling you that somewhere the circuit is open or reversed. Ask the child to describe precisely what is happening ("it doesn't go" won't do; "the back wheels turn but the front ones don't" will) and note in the notebook what you change and what result it gives. Change just one thing at a time: if you change three at once and it works, you don't know which one was the problem.

Praise the process, not the talent

"You're so clever" or "you've got an artist's hands" teach that the result depends on something you either have or don't have; when something fails, the child concludes they don't have it. Praise what depends on them: the strategy ("you measured twice before cutting"), the effort ("you tried four ways of holding the motor"), the improvement ("the second version goes twice as far") or the question ("what a good question, I hadn't thought of that"). Be specific and don't overdo it: children can spot automatic praise.

When to step in

Step in immediately, without discussion, if there is a safety risk: misuse of the craft knife, a hand in front of the blade, the hot glue gun pointing at skin, a battery getting hot, small pieces near the mouth of a child under 3, a chemical near the eyes. For everything else, use this scale, from least to most:

1. Wait and watch (1–2 minutes): many problems sort themselves out.
2. Open question: "What's happening?".
3. Hint: "Look at the check in step 5".
4. More specific hint: "Look at where the long leg goes".
5. Demonstration on a practice piece.
6. Doing it together: your hands guide theirs, or you do half.
7. Doing it yourself, explaining out loud, only if the step is beyond their physical ability or they have been stuck for more than 15 minutes.

Sharing out tasks by age

The ages are a guide: an experienced 7-year-old can work like a 9-year-old and vice versa. Look at fine motor skills (can they cut along a line? can they open and close a split pin?) more than age.

Safety in depth

The projects on this website are safe if you follow the pages, but they use sharp tools, heat, batteries, magnets and kitchen products. Almost all accidents in children's workshops come from four sources: craft knife cuts, minor burns from hot glue, batteries or magnets that end up in a small child's mouth, and products that splash into the eyes. This section explains each risk, how to prevent it and what to do if it happens. Each project page also adds its own specific warnings.

Risk table

Eye protection

Keep one pair of safety glasses per person in the workshop, including for the adult. They must also cover the sides ("wraparound" style), be marked EN 166 and have an adjustable strap; children's sizes cost €3–6. Prescription glasses don't protect against splashes from the side. They are compulsory for:

• Anything that splashes or fizzes: volcano, Vinegar and bicarbonate of soda rocket, Fizzy drink and sweets fountain, elephant toothpaste.
• Anything that shoots off: Lever catapult, Compressed-air rocket launcher, stretched rubber bands.
• Cutting wire or cables with pliers (the little bits go flying).
• Drilling.

Workshop first aid kit

• Sterile gauze pads (10) and paper surgical tape
• Plasters in various sizes
• Single-dose saline (for rinsing eyes and wounds)
• Antiseptic (chlorhexidine)
• Nitrile gloves (latex-free)
• Tweezers (for skewer splinters)
• Instant cold pack (for bumps)
• Round-ended scissors for the first aid kit
• Sheet with phone numbers: 112 and Toxicology 91 562 04 20
• In the classroom: list of the group's allergies

For burns, the best "first aid kit" is the cold tap: make sure there is one a few steps away.

Workshop rules (to read with the children)

Read them aloud in the first session, ask the children to explain them in their own words and put them up where everyone can see. With children aged 5–7, stick to the first five.

• Tools with the red hand symbol (craft knife, hot glue, drill) are used by an adult or with an adult right beside you.
• Nothing from the workshop goes in your mouth: not pieces, not batteries, not magnets, not mixtures, even if they come from the kitchen.
• Glasses on when something fizzes, splashes or goes flying.
• Scissors are passed closed, with the handle towards the other person.
• If something gets hot, smells strange or breaks: I let go, tell someone and step back.
• Batteries only. Never touch the mains socket.
• Before putting the battery in, I check the circuit with my finger.
• Everything goes back in its place at the end; small pieces go back in their jar.
• I wash my hands after using chemicals, even kitchen ones.
• I test rockets and catapults aiming at the target, never at a person.

Setting up the space and the materials box

A good home workshop doesn't need a room: a table, a big box and somewhere to leave projects to dry are enough. What really makes the difference is having everything ready before you sit down with the child. Every minute you spend looking for the scissors is a minute in which the child loses interest.

The work area

• Table at the right height: when seated, the child's elbows should be slightly above the table. If the table is high, a firm cushion on the chair; if their feet dangle, a box under their feet. For cutting with a ruler, standing is better.
• Protect the surface: an oilcloth, a large piece of cardboard (replaced when it gets damaged) or an A3 cutting mat. For chemistry, a baking tray with a rim: anything spilt stays inside.
• Light: natural, or a lamp that doesn't cast a shadow over the working hand (on the left for right-handers, on the right for left-handers).
• Socket for the glue gun at one end of the table, with the lead not crossing where people walk. If you use an extension lead, tape it to the floor.
• Drying area: a shelf, the top of a cupboard or an open cardboard box where projects can spend the night without anyone touching them. Label them with the name and date.
• Water nearby: a washing-up bowl or the sink for washing brushes and hands, and for cooling burns.
• Bin bag and recycling box to hand from the start, not at the end.

How to organise the materials box

The website suggests a workshop box with everything you need. Ideas that work:

• A large plastic box with a lid (about 40 × 30 × 20 cm) for tools and components, and a cardboard one for recycled materials.
• Small components in a compartment organiser (the kind used for screws or fishing tackle): one compartment per type of LED, one per resistor value (labelled with the value: the colour bands are very hard to read), clips, split pins, magnets, motors.
• Cardboard sorted by size and flattened, standing upright in a box like folders. Throw away any that is damp, crumpled or greasy.
• Clean, dry tubes and bottles, in a separate bag. Milk or juice bottles must be rinsed well or they smell within a week.
• One tray per child (a shoebox lid will do) so their pieces don't get mixed up with someone else's.
• A jar of salvageable parts: motors, LEDs, battery holders and wheels from dismantled projects. Almost all components can be reused ten times.

Recycled materials and what they are for

Recommended tools

The basic component kit

Buying components one at a time for each project is expensive and slow. This kit, bought once, covers the electronic and mechanical side of almost every project. Prices are approximate (online and electronics shops, 2026). The detailed list with links is in the shopping list.

Approximate total: about €80–125 including the basic tools (without multimeter or drill). Components only, about €70–95. For a class of 25 pupils, multiply the quantities of LEDs, clips, battery holders and syringes by 3 or 4, not by 25: pupils work in teams and components are reused.

Cardboard techniques

Cardboard is the main material in half the projects and the cause of half the failures: chassis that bend, wheels that wobble, joints that come unstuck. Almost all of it can be solved with half a dozen techniques you can learn in an afternoon. If you are going to build vehicles, machines or robots, this is the most important section of the manual.

Types of cardboard and when to use each

The direction of the flutes

Corrugated cardboard has two flat faces and, between them, a zigzag sheet (the flutes) that forms parallel channels. Look at the edge of any box: you will see the channels as little tunnels. Those flutes make the cardboard behave very differently depending on the direction:

• It bends easily along a line parallel to the channels (the flutes act as a hinge).
• It is very stiff if you try to bend it along a line perpendicular to the channels (you would have to crush every tunnel).

In practice: in a long piece that must not sag (a chassis, a catapult arm, a crane jib), the channels should run along the length of the piece. In a piece that has to be folded (the walls of a box), the fold line should be parallel to the channels. The templates on the website have a "flutes" arrow showing how to place them.

Measuring and marking

• Measure in millimetres, not in centimetres with decimals: "85 mm" is easier to read than "8.5 cm". The project pages always use mm.
• Mark with a well-sharpened HB pencil, using a dot or a small "V" rather than a thick line. Join the dots with the ruler.
• For a right angle, place a set square (or the edge of a sheet of paper, whose corners are at 90°) against a straight edge of the cardboard. Don't trust the edges of a box: they are usually crushed or crooked. Cut a straight edge first and measure everything from it.
• When you need to cut several identical pieces (four wheels, two sides), make one and use it as a template to mark the rest.
• Measure twice, cut once. With children aged 7–10, let the child check the measurement before the adult cuts.

Cutting straight

Scissors cut thin card and single-wall corrugated cardboard well in small, curved pieces. Cut with the part of the blade nearest the pivot (it has more force) and turn the cardboard, not the scissors. For cutting corrugated cardboard straight, a craft knife gives a much better result:

1. Place the cardboard on the cutting mat and the metal ruler on the line, with the ruler over the piece you want to keep (if the knife slips, it spoils the offcut, not the piece).
2. Hold the ruler with your hand open, fingers away from the edge and resting on the middle of the ruler.
3. First pass light, without pressing, just to cut the top face and mark the groove.
4. Second pass along the same groove, a little firmer: it cuts the flutes.
5. Third pass to cut the bottom face. Three light passes give a clean, vertical cut; one forceful pass crushes the flutes and the blade wanders.
6. Hold the craft knife at a low angle (about 30–45° to the table) and the blade perpendicular to the cardboard, not tilted to either side.

Circles

• To draw: a compass, or a round object of the right size (30 mm bottle cap, glass, roll of tape, 120 mm CD). The website template always marks the centre.
• To cut with scissors: first cut out a square around it, then cut off the corners (octagon) and then round it off with small cuts. Turn the cardboard, not your hand.
• To cut with a craft knife: many short, tangential cuts, turning the cardboard. Don't attempt one continuous curve.
• To sand: a sheet of 120-grit sandpaper glued to a wooden block evens out irregular edges.
• To make the centre hole: an awl on the marked centre, with the disc resting on modelling clay. Widen it by twisting a pencil if needed.

Folds: scoring, valley and mountain

A fold that hasn't been scored ends up rounded and crooked. Before folding, score the line by pressing with an empty ballpoint pen, the tip of closed scissors or the back of the craft knife blade, always against the ruler. On thin card one pass is enough; on corrugated cardboard, press until the face is slightly flattened. To fold corrugated cardboard against the flutes, the adult can make a half-depth cut (top face only) on the outside of the fold.

The templates use two types of fold line, as in origami:

Joints

Choosing how to join two pieces matters as much as cutting them well.

Cross slots: the slot should be as wide as the cardboard is thick (3–4 mm in single-wall corrugated; two craft knife blades held apart, or two parallel cuts and pull out the strip). If it is wider, the pieces wobble; if it is narrower, the flutes get crushed and it breaks. The length is half the height of the piece, from the top on one and from the bottom on the other.

Reinforcements

• L-shaped corners: a 20 × 40 mm strip of cardboard folded to 90° and glued inside a corner doubles its strength.
• Triangles (gussets): a 30 × 30 mm right-angled triangle of cardboard glued into the angle between a wall and the base stops the wall from leaning. It is the most effective reinforcement for towers, cranes and bridges (see structures).
• Laminating: glue 2 or 3 layers of cardboard with the flutes crossed (one with the channels horizontal, the next vertical). The result is stiff in every direction, like plywood. Use it for wheels, gears, levers and bases. Glue them with PVA glue and leave them to dry under a weight (a couple of books) so they don't curl.
• Edges: an exposed corrugated edge gets crushed and frays. Cover it with masking tape.
• Tubes: a kitchen roll tube standing upright can hold a great deal of weight. Use it as a column.

Axles and wheels

This is the technique that makes the most projects fail. The rule is: the wheel and the axle turn together; the axle turns inside a bearing that is fixed to the chassis. If the axle (the wooden skewer) goes straight through a hole in the cardboard, the cardboard wears, the hole gets bigger and the friction slows the vehicle down. The solution is a straw.

Clearances (difference between the diameter of the axle and that of the hole):

How to centre a bottle cap. If the hole is not in the centre, the wheel "limps" and the vehicle bounces. A reliable method: draw round the cap on paper, cut out the circle and fold it in half twice; the corner of the fold is the centre. Put the circle inside the cap and mark the centre with the awl. The adult drills it with a 2.5 mm bit or with a nail heated in a flame (held with pliers). The website templates include a centring guide for 30 mm bottle caps.

Checks before you carry on:

• The skewers are straight: roll them on the table; if they "wobble", replace them.
• The two straws are parallel to each other and perpendicular to the long axis of the chassis (use the set square).
• The skewer turns freely inside the straw: blow on a wheel and it should spin several times on its own.
• The wheels don't touch the chassis or the straw (2–3 mm clearance).
• Seen from the front, the wheels are upright and don't wobble as they turn.
• All four wheels touch the ground at the same time.

Hinges

• Tape hinge: place the two pieces side by side with a 2 mm gap and put masking tape or cloth tape on both faces. Quick and good enough for lids and doors.
• Living hinge: on corrugated cardboard, the adult cuts only the top face and the flutes, without cutting the bottom face. The remaining face acts as the hinge. Use it for folds that are opened and closed only a few times.
• Pin hinge: two pieces of straw glued to one piece and one to the other, alternating, with a wooden skewer running through them. Very durable; it is the one used in the Castle drawbridge.

Joints with split pins

The split pin (the "drawing pin" with legs that open out) is the easiest way to make a joint that turns: arms, legs, levers, the Mechanical hand with tendons or the Syringe digger arm.

1. Mark the pivot point on both pieces, at least 10–12 mm from the edge.
2. Pierce with an awl over modelling clay and widen to about 4 mm by twisting a pencil: the joint should turn without forcing.
3. Lay the pieces on top of each other, push the split pin through and open the legs in opposite directions, flat against the cardboard.
4. If it is loose, put a thin-card washer between the pieces. If it is stiff, loosen the legs a little.
5. For joints that do a lot of work, reinforce the hole with a circle of masking tape on both faces before piercing.

Cardboard pulleys and gears

Pulley: glue together three laminated discs: two large ones on the outside (for example, 40 mm) and a smaller one in the middle (30 mm). The gap between the two large ones is the groove where the string or rubber band runs, and it stops it slipping off. With bottle caps: glue two caps together base to base, open ends facing out; the join forms the groove.

Gears from the cardboard flutes: the most useful trick for homemade gears. Cut a strip of corrugated cardboard 10–15 mm wide with the channels running across the strip, dampen one face slightly and peel that flat face off: the flutes are left exposed. Wrap the strip, flutes facing out, around a laminated cardboard disc and glue it with PVA glue. The channels act as teeth, all the same (about 4 mm pitch in single-wall cardboard), and they mesh with any other gear made the same way. It is the technique used in the Crane with gear wheel and pinion.

Template gears: cut out the printed gear glued onto two laminated layers. Teeth less than 5 mm wide break in corrugated cardboard: for small gears use 2 mm greyboard. Space the axles of two gears exactly as the project page says (the sum of the two radii): if they are too close they jam; too far apart, they skip.

How to use the 1:1 templates

1. Print at 100% and check that the red line in the header measures 50 mm (see How to use this website).
2. Cut out the template leaving a margin of about 5 mm around each piece.
3. Method 1, gluing: run a glue stick over the back of the paper (only on the corners and the centre if you want to peel it off later) and stick it onto the cardboard, following the flute direction arrow. Cut through paper and cardboard together along the solid line. It is the most precise method.
4. Method 2, tracing: hold the template in place with masking tape and go over the outline with an awl or a ballpoint pen, pressing hard: it leaves a mark on the cardboard. Or cut out the template and draw round it in pencil.
5. Method 3, reusable template: for pieces you will make many times (in the classroom), glue the template onto 2 mm greyboard, cut it out and use it to draw round.
6. Transfer the markings to the cardboard as well: holes (with an awl through the paper), fold lines (scoring them) and the letter of each piece.

3D printing

The pieces for the projects can also be 3D printed. The files are prepared for an Anycubic Kobra 3 with ACE Pro (250 × 250 mm bed) and PETG, although they work on any printer with a bed of that size. On each project page, the "3D printer" section tells you whether it is worth printing the project and which pieces.

• Recommended: the project is better in PETG. This is usually for one of these reasons: it has mechanisms that wear (gears, pulleys, cranks, joints), it has to withstand force (syringes, rubber bands, taut strings), it gets wet or it has to be precise (holes, slits, distances).
• Some pieces: only some pieces benefit from PETG. The rest are made of cardboard or paper because they don't fit on the bed, because they have to be light or because they are paper (templates, scales, drawings).
• Better in cardboard: it isn't worth printing. The project may need to be very light (balloons, air, wind), it may have to float or withstand heat, or it may use only paper.

What you download

• Plates (.3mf): each file is a complete bed with the pieces and their copies already laid out, without overlaps. Open it in Anycubic Slicer Next (or any slicer) and slice it without moving them. Long, narrow pieces that only fit diagonally go on their own plate, already rotated 45°.
• Individual pieces (.stl): for reprinting just the one that has broken or gone missing.
• Don't scale the models in the slicer: the hole clearances and the hinge thickness are calculated in real millimetres.

How the cardboard has been converted to PETG

The top face of the print is the face you see on the template. The following are not printed: paper, clear or reflective plastics, polystyrene, materials that are not cardboard (straws, skewers, wire, rubber bands...) and pieces that don't fit on the bed: those are made as the project page says.

Starting settings (PETG on the Kobra 3)

Start with the PETG profile in Anycubic Slicer Next for the Kobra 3 (0.4 mm nozzle) and check these values:

The grams and times given on each project page are an estimate made with these settings. The slicer will give you the exact figure.

The ACE Pro: colours and drying

• One colour per plate. Each piece on the plate is an object with its letter and name, so you can assign it a colour. But all the pieces build up at the same time, layer by layer: with two colours on the same plate, the printer would change filament on every layer and purge a lot of plastic. Choose one colour for the whole plate and change spools in the ACE Pro before the next plate.
• Colours with a purpose. For light and shadow projects (spectroscope, camera obscura, shadow theatre, planetarium, zoetrope) use black or dark, opaque PETG: light colours let light through the plastic. For those that diffuse light (smart house, lava lamp), white or translucent PETG.
• Drying. PETG absorbs moisture from the air. If it crackles while printing, leaves lots of strings or the hinges break on the first bend, dry it in the ACE Pro (at its maximum temperature, for about 4–6 hours) and store it with desiccant.

Hinges: bending without breaking

1. Wait until the bed has cooled completely: the PETG comes away on its own or by flexing the plate slightly.
2. Bend each hinge slowly the first time, resting the piece on the edge of the table. A "valley" fold bends towards the top face (the groove closes) and a "mountain" fold bends backwards.
3. Bend and unbend it 3 or 4 times before gluing. A well-printed PETG hinge withstands hundreds of bends.
4. If a hinge cracks on the first bend, the filament is damp or the fan is set too high (see above).

Assembly: clearances, axles and glue

• Axles: 3 mm skewers go in with 0.3 mm of clearance and turn without rubbing. If one is a tight fit, ream the hole by twisting a 3.5 mm drill bit with your fingers, without a drill.
• Gluing PETG: PVA glue and glue sticks don't grip. Use superglue (cyanoacrylate) (applied by the adult) or hot glue, and sand the contact area first with 120-grit sandpaper. Double-sided foam tape also works for mounting onto cardboard.
• Burrs: the adult removes them with a craft knife; children can smooth the edges with sandpaper.

What changes compared with cardboard

• Weight. At the same thickness, a PETG piece weighs between 3 and 5 times as much as a cardboard one (the project page gives both weights). In things moved by little force (balloons, solar motor, wind, vibration) this is a problem. In things that turn or get knocked it can be an advantage: a heavier drum turns more smoothly and a heavier domino falls more reliably.
• Water: PETG doesn't soften in water, but it doesn't float (1.27 g/cm³). A boat needs a closed hull.
• Heat: PETG starts to soften at around 75–80 °C. No pieces inside the solar oven, with boiling water or in a car in the sun in summer.
• Durability: a PETG piece can last for years. It is worth it for things that are used again and again (launchers, gauges, frames, mechanisms).

Printer safety

• An adult operates the printer. The nozzle reaches about 240 °C and the bed 80 °C: children watch from outside and don't touch the bed until it has cooled down.
• Print in a ventilated room and not in the children's bedroom.
• Small pieces (washers, spacers, counters) can be swallowed: keep them out of reach of children under 3.

Electricity without fear

You don't need to know electronics to make the electrical projects on this website. You need to understand five ideas (circuit, voltage, current, polarity, short circuit) and know how to look for a fault methodically. Everything runs on 1.5 to 9 V batteries, which are not dangerous to people. The only real danger is the mains socket, which is never used.

The circuit: the water analogy

Picture a circuit as a closed water circuit: a pump pushes the water through a pipe, the water turns a waterwheel and returns to the pump.

The key idea: electricity only flows if the path is closed, from the positive terminal (+) of the battery, through the components, to the negative terminal (−). If there is a break anywhere (a loose wire, a crocodile clip that isn't biting, an open switch), nothing works. That is why it is called a circuit: it is a circle.

Batteries: which to use

Why we put batteries in battery holders: they connect with firm wires, they are easy to replace, and it is much harder to short-circuit them by accident. Two AA batteries in a 2-battery holder are in series: their voltages add up (1.5 + 1.5 = 3 V). The red wire is +, the black one is −. If the battery holder has a switch, turn it off while you are building.

Polarity

Some components work the same whichever way round they are connected (resistors, bulbs, motors, although a motor changes direction). Others only work one way round: LEDs, active buzzers and solar panels. This is called polarity. A convention worth always following: red = positive (+), black = negative (−). If all the + wires are red, mistakes can be spotted at a glance.

The LED

An LED (light-emitting diode) is a small lamp that uses very little power, but it has two requirements:

1. It goes one way round. The long leg (anode) goes towards the + of the battery; the short leg (cathode) towards the −. If the legs have been trimmed, look at the rim of the casing: the flat side is the short leg (−). Inside a clear LED, the larger piece is usually the −. If you put it in the wrong way round it doesn't break: it simply doesn't light up. Turn it round.
2. It needs a resistor to limit the current. An LED doesn't limit the current by itself: if you connect it directly to a battery with a higher voltage than it needs, so much current flows that it heats up and burns out in seconds (or weakens and lasts much less). The resistor can go before or after the LED: it makes no difference.

Which resistor to use. Values calculated for about 10–15 mA, enough for the LED to shine brightly and last a long time. If you don't have the exact value, use the next one up: the LED will be slightly less bright, with no danger.

Green LEDs can be 2 V (yellowish green) or 3 V (bright green). Start with the resistor from the red column; if it doesn't light up or is very dim, try the one from the blue column.

For adults, the formula: R = (V_battery − V_LED) / I. For example, a red LED (2 V) with a 9 V battery and 15 mA: R = (9 − 2) / 0.015 = 467 Ω → 470 Ω. The resistors in the kit are identified by their colours (4 bands): 100 Ω = brown-black-brown; 220 Ω = red-red-brown; 470 Ω = yellow-violet-brown; the fourth, gold band is the tolerance. Keep them in labelled compartments and you won't have to read colours.

DC motors

The motors in the kit (type 130, 3–6 V) start turning as soon as they receive current. Three things worth knowing:

• The direction of rotation depends on how you connect them. If you swap the two wires, it turns the other way. There is no "wrong" way: if the fan blows backwards or the car goes in reverse, swap the wires.
• They turn very fast and with little force (about 6000–9000 revolutions per minute with no load). To drive a vehicle you need to reduce the speed with a pulley or gears (see mechanisms); connected directly to a wheel, the motor stalls.
• They need quite a lot of current to start (0.3–1 A). That is why a type 130 motor won't start with a mini solar panel (which gives 100–200 mA) and doesn't work well with a 9 V battery. Solar projects need a low-start-current "solar motor" (RF-300 or similar), which turns with 25 mA, as in the Solar car with belt drive. A motor that hums but doesn't turn is asking for more current than it is getting, or is mechanically jammed.

Active and passive buzzers

An active buzzer has a small circuit inside that makes it beep as soon as it receives direct current: it is the one needed for projects such as the Door alarm with buzzer or the Push-button telegraph. It has polarity (long leg or red wire to +) and usually comes with a sticker that is meant to be removed after soldering; you can leave it on, it just muffles the sound. A passive buzzer is just a small speaker: with a battery it only "clicks" when connected. If you are buying one, look for an "active buzzer 3–5 V". To tell them apart: connect it to 2 AA batteries; if it beeps continuously, it is active.

Homemade switches

A switch is nothing more than two metal contacts that touch or separate. Three versions you can make in 5 minutes:

• Paperclip and split pins: two split pins pushed into a piece of cardboard about 25 mm apart; a wire under each one (wound round the legs). A metal paperclip (with no plastic coating) is hooked on one of them and swings round to touch the other. It is the switch in the Tube torch with a switch.
• Aluminium foil: two 2 × 2 cm squares of aluminium foil stuck down with tape, each with the stripped end of a wire underneath. When they are pressed together (for example, when a door closes or someone steps on a mat) the circuit is closed. It is the basis of the Door alarm with buzzer.
• Push button: a 2 × 8 cm strip of cardboard folded in half, with aluminium foil or copper tape on the inside face of each half, and a small piece of craft foam or cardboard between them at the end by the fold so that they spring apart on their own. When you press, the contacts touch; when you let go, they open. Ideal for the Push-button telegraph.

Connections without soldering

• Crocodile clips: the quickest way to test. Clip onto the metal, not the plastic of the wire. Cheap clips sometimes have the wire badly attached inside: if a circuit fails, swap the clip.
• Twist and tape: strip 15 mm from each wire, cross the strands and twist them together 4–5 turns, fold the joint back and cover it with electrical tape. To join a wire to an LED leg, wind the wire 3 times round the leg.
• Split pins as terminals: wind the wire round the legs before opening them out.
• Copper tape: tracks stuck onto cardboard or paper. If the tape doesn't have conductive adhesive, don't cut the tape at corners: fold it over on itself. At joins, put a piece on top and press down.
• Conductive dough: homemade dough made with salt conducts; dough made with sugar insulates. See Circuits with conductive dough.
• Stripping wires: use wire strippers; if you don't have any, the adult scores the insulation with scissors while turning the wire (without squeezing) and pulls it off. The copper strands should be left shiny and uncut.

Series and parallel

When there are two components (two LEDs, two motors) they can be connected in two ways:

The short circuit

A short circuit is a path with nothing to slow the current down: a wire (or aluminium foil, or a clip) that directly connects the + and − of the battery. All the battery's energy turns into heat in the wire and in the battery itself. With AA batteries you can feel it getting hot within 5–10 seconds and the battery runs flat; with lithium batteries or several batteries it can even cause burns. Signs: the wire or the battery holder gets hot, nothing works, the battery runs down very quickly. Prevention: trace the route of the circuit with your finger before putting the battery in ("a wire comes out of the +, goes through the resistor, through the LED and back to the −") and check that no bare wires are touching.

Troubleshooting a circuit that doesn't work

Follow this order. In the vast majority of cases the fault is in the first three steps.

1. Is it switched on? The battery holder switch set to ON, the homemade switch closed.
2. Are the batteries all right? Fitted according to the + and − markings in the battery holder; new or tested in another device (or with the multimeter: more than 1.3 V each).
3. Are the LEDs, buzzers and panels the right way round? Long leg towards the +. If in doubt, turn it round: it won't break.
4. Is the circuit closed? Follow the path with your finger from the + to the −. Look for clips biting plastic instead of metal, wires twisted together without being stripped, split pins not touching the wire.
5. Is there a short circuit? If something gets hot, disconnect it and look for two wires touching or a path that bypasses a component.
6. Is a component faulty? Test each part on its own directly with the battery holder (the LED with its resistor). Swap the crocodile clips for different ones.
7. Is it the right resistor? A 470 Ω one with 2 AA batteries leaves the LED very dim, almost invisible in daylight.
8. Divide and conquer: if the circuit is large, build just the first part (battery + one LED) and keep adding parts until it stops working. The last part added is the suspect.

Basic multimeter (optional, for adults and over-11s)

A €10–15 multimeter turns troubleshooting into a 30-second check. You only need two functions:

• Measuring batteries: set the dial to "V⎓" (direct current), 20 V range. Red probe on the +, black on the −. A new AA battery reads 1.5–1.6 V; below 1.2 V it is too flat for motors. If you get a negative number, you have the probes the wrong way round (no harm done).
• Continuity (the beep): dial set to the speaker or diode symbol. With no battery in the circuit, touch the two ends of a wire, a clip or a closed switch: if it beeps, current can pass. Ideal for finding broken clips and badly stripped wires.

Don't use the current ranges (A, mA) unless you know what you are doing: if you measure the current directly across the terminals of a battery, you create a short circuit and can blow the multimeter's fuse. And never, under any circumstances, bring it near a mains socket with the children.

Mechanisms and physics

Behind every vehicle, crane or catapult there is a handful of physical principles that keep coming up. Knowing them lets you explain why a project works and, above all, why it doesn't. For each concept you'll find an explanation for you, a sentence you can say word for word to a 6–8-year-old, and the projects on the site where it appears.

Levers

For the adult. A lever is a rigid bar that turns about a support point (the fulcrum). The law of the lever says that effort × distance from the fulcrum = load × distance from the fulcrum. If you push three times further from the fulcrum than the load is, you need a third of the force, but your end travels three times as far: a lever doesn't give you free energy, it trades force for distance. Depending on where the fulcrum, the effort and the load are, there are three classes:

For the child: “With a lever you can lift very heavy things if you push far away from the point where it rests. The further away you push, the less force you need.”

Where it appears: Lever catapult (try moving the fulcrum and measure the launch distance), Castle drawbridge, Tabletop pinball machine (the flippers), Arcade claw machine, Push-button telegraph (the key).

Pulleys

For the adult. A fixed pulley (attached to the ceiling or the crane's jib) doesn't reduce the force, it only changes its direction: you pull down to lift something up. A movable pulley (which rises with the load) shares the weight between two lengths of string and halves the force, in exchange for pulling twice as much string. Combining pulleys (a block and tackle) multiplies the effect. Two pulleys of different sizes joined by a belt (a rubber band) work like gears: the gear ratio is the ratio of the diameters. In the Solar car with belt drive, the motor shaft (about 3 mm with its sleeve) drives a 20 mm pulley: about 7 turns of the motor for every turn of the wheel, and 7 times more force.

For the child: “A pulley is a wheel with a groove that the string runs through. If the string goes down on one side, it goes up on the other. With several pulleys you can lift heavy things with very little force, but you have to pull lots of string.”

Where it appears: Pulley crane with winch, Solar car with belt drive (belt), Castle drawbridge, Tabletop chain reaction.

Gears and gear ratio

For the adult. Two meshed gear wheels turn in opposite directions. If one has 12 teeth and the other 24, when the small one makes a full turn it has pushed 12 teeth of the big one, which has only gone half a turn. The gear ratio is 12:24 = 1:2. When the small one drives the big one (reduction), the output shaft turns more slowly but with more torque (turning force); the other way round (step-up), it turns faster with less force. It's the same as changing gear on a bicycle. To make the last gear turn the same way as the first, you add one in between (an “idler”), which doesn't change the ratio.

For the child: “The teeth on one wheel push the teeth on the other. If the big wheel has twice as many teeth, the small one has to go round twice for the big one to go round once. The big one goes more slowly, but it's stronger.”

Where it appears: Crane with gear wheel and pinion, Solar car with belt drive, Cup wind meter (for counting turns).

Wheels and axles

For the adult. A wheel greatly reduces friction compared with dragging an object: instead of sliding over the floor, the wheel rolls, and the friction is concentrated at the axle, which is small and can be lubricated or put in a bearing. A large wheel gets over obstacles better and travels further per turn; a small one needs less force to turn. The vehicle only goes straight if the two axles are parallel and the wheels are the same.

For the child: “It's much easier to push a box with wheels than to drag it. The wheels turn, and the stick in the middle, the axle, turns inside a tube so it doesn't get stuck.”

Where it appears: all the vehicles: Solar car with belt drive, Rubber band car, Balloon-powered car; also the Four-blade pinwheel and the Cup wind meter. Technique in Cardboard techniques.

Friction

For the adult. Friction is the force that resists sliding between two surfaces. It depends on the materials and how hard they press together, and hardly at all on the contact area. It's the enemy at the axles (it slows the vehicle down and drains the battery) and the ally at the wheels (without friction against the floor they would skid) and in joints. That's why we put rubber bands on smooth bottle caps as tyres and use straws as bearings for the axles. Air causes friction too: the CD hovercraft glides on a cushion of air that almost eliminates friction with the table.

For the child: “When two things rub together, they slow each other down. That's why you slip in socks and not in trainers. We want the wheels to grip the floor, but the axle to turn without being slowed down.”

Where it appears: Rubber band car, CD hovercraft, Marble maze tray, Tabletop pinball machine.

Hydraulics with syringes

For the adult. If you join two syringes with a tube full of water and push one plunger, the other comes out. Water is practically incompressible: all the volume that goes in at one end comes out of the other instantly. The system also works with air, but air compresses like a spring: there is a delay, the movement is spongy and it loses force. That's why we use water. If the syringes are different sizes, the pressure is the same throughout the liquid (Pascal's principle), but the force is proportional to the area of the plunger: pushing a 10 ml syringe (plunger about 15 mm) against a 20 ml one (about 20 mm), the big one pushes with about 1.8 times more force, but moves 1.8 times less. That's how car brakes and diggers work.

For the child: “You can't squash water. If you push water along a tube, it comes out at the other end and pushes whatever is there. Air does squash, like a cushion, which is why with air it moves in jerks.”

How to fill it without bubbles: put both syringes and the tube under water in a washing-up bowl (a little food colouring helps you see the bubbles). Fill one syringe completely, leave the other with its plunger pushed in, join the two with the tube under the water and lift the whole thing out. If there are bubbles left, push them towards one syringe held with its tip upwards, undo the tube and push them out.

Where it appears: Syringe digger arm, Hydraulic goods lift.

Centre of gravity

For the adult. This is the point where we can imagine all the weight of an object is concentrated. An object stays upright as long as the vertical line through its centre of gravity falls within its base. The lower it is and the wider the base, the more stable the object. A hanging object comes to rest with its centre of gravity directly below the point it hangs from: that's why toy tightrope walkers have weights at the ends of their arms, below the support point.

For the child: “Everything has a point where it balances. If you put the weight low down and the feet wide apart, it's very hard for it to fall over.”

Where it appears: Table that seems to float, Six-legged jointed spider, Spaghetti and marshmallow tower, Gliding paper aeroplane (the paperclip on the nose moves the centre of gravity forward and stabilises it).

Structures and triangles

For the adult. A rectangle of bars joined at the corners is easily deformed (it turns into a rhombus) because its angles can change. A triangle isn't: with three fixed sides, its shape is unique. That's why cranes, electricity pylons and bridges are full of triangles. In cardboard, a diagonal or a triangular corner brace in each corner turns a floppy structure into a rigid one. Also, parts work better in tension or compression than in bending: a flat A4 sheet holds nothing up; folded like a concertina or rolled into a tube, it holds up books.

For the child: “Push a square made of sticks: it bends out of shape. Now try a triangle: it doesn't move. Triangles are the strongest shape.”

Where it appears: Spaghetti and marshmallow tower, Accordion paper bridge, Pulley crane with winch, Crane with gear wheel and pinion.

Elastic and potential energy

For the adult. Energy isn't created, it's transformed. A stretched rubber band, a bent plastic spoon or a blown-up balloon store elastic energy; an object held up high stores gravitational potential energy (more the higher and heavier it is). When they are released, that energy turns into movement (kinetic energy). In every transformation some is lost as heat and sound through friction, which is why a marble never climbs back up to the height it started from. “Friction-powered” cars store energy in a flywheel that spins fast.

For the child: “When you stretch a rubber band, you're storing energy inside it. When you let go, the energy comes out and launches the ball. A marble up high also has energy stored: when it falls, it uses it to run.”

Where it appears: Lever catapult, Rubber band car, Jumping paper frog, Marble maze tray, Tabletop chain reaction.

Action and reaction

For the adult. Newton's third law: when one body pushes another, it receives an equal push in the opposite direction. A balloon that pushes air out backwards is pushed forwards; a rocket doesn't “lean” on the air, it pushes the gases (or the water) it throws out. The more mass it throws out and the faster, the greater the thrust: that's why the bottle rocket with a little water goes much further than with air alone.

For the child: “If you push something, that something pushes you back. The balloon pushes the air backwards, and the air pushes the balloon forwards.”

Where it appears: Balloon-powered car, Balloon rocket on a string, Compressed-air rocket launcher, Vinegar and bicarbonate of soda rocket, Pop-pop sailing boat.

Inclined plane and screw

For the adult. Moving a weight up a ramp takes less force than lifting it straight up, in exchange for covering more distance. A screw is an inclined plane wrapped around a cylinder: as you turn it, whatever is in the thread rises little by little. Archimedes' screw lifts water by turning a tilted helix.

For the child: “It's easier to walk up a gentle slope than to climb up a wall, even though the path is longer.”

Where it appears: Spiral water lifter, Marble maze tray, Tabletop chain reaction.

Chemistry and science experiments

An experiment isn't a magic trick you repeat until it gets boring. It's a question you answer by testing. The difference between “making a volcano” and “experimenting with a volcano” is small in time (10–15 minutes more) and huge in what the child learns. This section explains how to turn any project into a real experiment.

The method, step by step

1. Question. Something specific that can be tested. Not “how does the volcano work?”, but “does more foam come out if I add more vinegar?”. The project pages include questions in the “Experiment” section; the best ones are the ones the child comes up with.
2. Hypothesis (prediction). What they think will happen and why, before testing it: “I think more foam will come out because there's more liquid to react”. Get them to write it down or draw it. Getting the prediction wrong isn't a mistake: it's the most interesting part, because it means they've learnt something they didn't know.
3. Fair test: change only one thing. If you change the vinegar and the bicarbonate of soda too, you won't know which of the two made the difference. There are three kinds of variable:
   • The one I change (independent variable): the amount of vinegar: 25, 50 and 75 ml.
   • The one I measure (dependent variable): the height of the foam in the glass, in cm.
   • The ones I keep the same (control variables): the same spoonful of bicarbonate of soda, the same glass, the same vinegar temperature, the same colouring, the same way of pouring it in.
4. Measure. With numbers, not impressions. “Loads came out” can't be compared; “it rose 6 cm” can. Use a ruler, tape measure, stopwatch, syringe or kitchen scales.
5. Repeat 3 times. No experiment comes out the same twice. Repeat each test 3 times and work out the average (add them up and divide by 3). If one of the three is very different from the other two, think about what might have happened (did the catapult move?) and repeat it.
6. Conclusion. Answer the original question using the data: “With more vinegar more foam comes out up to 50 ml; with 75 ml it's about the same because there's no bicarbonate of soda left to react”. And compare it with the hypothesis: were you right? Why not?
7. New question. Every conclusion opens up another: “What if I add more bicarbonate of soda with the 75 ml?”.

By age

The inventor's notebook

A notebook (hardback if possible, so it can be used on the table) where the child records each project. It lets them see their progress, feeds into the portfolio and helps them remember what worked. One page per project with these headings; with young children, the adult writes down what they say and the child draws.

Measuring: quantities and equivalents

The project pages give quantities in ml and g. If you don't have scales or a measuring jug, these equivalents are good enough for the experiments (level spoonfuls, levelled off with a finger or a knife):

• A 20 ml syringe without a needle is the most accurate and cheapest liquid-measuring tool in the workshop. Use it for small amounts (hydrogen peroxide, soap, colouring).
• Kitchen scales accurate to 1 g: for salt and sugar in the crystals (Cubic salt crystals, Rock candy on a string), where the amount matters.
• Yeast: Giant yeast foam needs dried baker's yeast (a 5–11 g sachet depending on the brand). Baking powder is no good: it's bicarbonate of soda with an acid.
• Temperature: “lukewarm water” = 35–40 °C (you can comfortably put your finger in it). “Hot tap water” = 45–55 °C. Crystals need very hot water (heated by the adult, not boiling, and handled by the adult).
• Time: the stopwatch on a mobile phone. One person says “go” and another times it.
• Distances: a dressmaker's tape measure or a builder's tape. Stick masking tape on the floor every 50 cm to measure launches and runs quickly.

Kitchen products: what each one does

Waste and cleaning up

• Vinegar, bicarbonate of soda, salt, sugar, 3 % hydrogen peroxide, soap: down the sink with plenty of water.
• Oil: never down the sink. Keep it in a bottle and take it to the used cooking oil bin or the recycling centre.
• Slime, glue, modelling clay, recycled paper: in the rubbish bin, not down the sink (they block the pipes). Wipe containers out with kitchen roll first, then wash them with water.
• Batteries: in a battery recycling bin (supermarkets, electronics shops). Never in the rubbish.
• LEDs, motors, panels, broken wires: these are electronic waste. Collect a few and take them to the recycling centre. Try to reuse them first.
• Cardboard and paper: in the paper recycling bin, free of hot glue and tape if possible.
• Wet PVA glue: comes off with water. Dried, on clothes: soak in lukewarm water and rub. On the table: scrape it off.
• Hot glue on the table: let it cool completely and lift it off with a spatula or a card.
• Food colouring on hands: soap and a pinch of bicarbonate of soda, rubbing well. Use nitrile gloves if it matters.
• Poster paint: washes off with water while still wet. On clothes, cold water (hot water sets some pigments).

Spend the last 10 minutes of each session tidying up with the children: each one cleans their tray, puts the small parts back in their pot and takes the waste to the right place. It's part of the work, not a punishment.

Planning a session

A well-planned session has an opening that hooks them in, a stretch of uninterrupted building time, a moment for testing and improving, and a wrap-up where you talk about what has been learnt. Without structure, a session tends to turn into 70 minutes of rushed building, 5 of testing and no reflection at all: exactly the opposite of what you want.

Structure of a 60–90-minute session

30-minute version

For a weekday afternoon or half an hour of form time. Choose a difficulty 1 or 2 project with a time on the project page of 30 min or less, or split a longer one.

• Beforehand: everything ready in the tray, tricky pieces pre-cut, templates already stuck onto the cardboard.
• Minutes 0–3: the challenge or the question.
• Minutes 3–20: building.
• Minutes 20–27: test, one improvement, second test.
• Minutes 27–30: one sentence each: “Today I found out that...”. Tidy up.

Projects that fit well into 30 minutes: Balloon rocket on a string, Two-sided spinning disc that merges pictures, Moving colours in milk, Raisins that rise and sink, Paperclip floating on water, Gliding paper aeroplane, One-sided strips, Foaming volcano.

To split a long project into 30-minute sessions, divide it at the drying times: session 1, structure and gluing; session 2, axles, wheels or circuit; session 3, testing, improving and decorating.

Example of a full session: balloon-powered car

Project: Balloon-powered car. Group: 4 children aged 8–9 and one adult. Length: 75 minutes. Aim: build the car, get it to travel at least 3 metres and test one variable (how much the balloon is blown up).

Preparation (25 min, the day before).

• Print 4 copies of the templates at 100 % and check the red 50 mm line at the top of each sheet.
• Prepare 4 trays with: a 16 × 9 cm piece of cardboard, 4 matching bottle caps, 2 wooden skewers, 3 straight straws, 2 balloons, 2 rubber bands, tape, template.
• Drill the 16 bottle caps (adult, with a 2.5 mm drill bit) using the centring guide.
• Stick a start line on the corridor floor and marks every 50 cm up to 6 m.
• Prepare a 20 cm strip of cardboard to measure the balloon's diameter, a tape measure, notebooks and pencils.
• Build a test car and check that it works.
• Ask about latex allergies.

Minutes 0–7. Hook. The adult blows up a balloon, lets it go and it flies round the room. “Why did it move? Which way was the air coming out and which way was the balloon going?”. Listen to the answers without correcting. “Today we're going to tame that balloon: we'll put it on a car so it goes in a straight line. How many metres do you think it'll go?”. Each child writes their guess in their notebook.

Minutes 7–10. Rules. “There are balloons today: if one bursts, you give me all the pieces. Nobody puts a piece of balloon in their mouth. Cars are released in the corridor, never pointed at anyone”.

Minutes 10–13. Plan. Look at the diagram on the project page together and find: chassis, bearings, axles, wheels, nozzle. “What do you think the straw under the chassis is for?”.

Minutes 13–43. Building. Each child builds their car following steps 1 to 8 in Workshop mode (one screen for everyone or the printed project page). Planned adult input:

• Step 3: check that the straws are parallel (this is the most common mistake). Ask: “How do you know they're parallel?”.
• Step 5: the adult puts the dot of hot glue on the bottle caps. Check the 2 mm clearance.
• Step 6: help hold the balloon while the child winds the rubber band round.
• Minute 28: “15 minutes of building left”. Minute 38: “5 minutes left”.
• Whoever finishes first: blow on the wheels and count how many turns they make on their own. If it's fewer than 2, find what's rubbing.

Minutes 43–63. Testing and improving. In the corridor. First test of each car with the balloon at 20 cm (the adult blows it up or supervises if they're under 8). Measure and record. Questions: “Did it go straight? What could you change?”. Each child makes one adjustment (straighten the nozzle, free a wheel that's rubbing) and tries again. Then the experiment: each child tests their car with the balloon at 10, 15 and 20 cm in diameter, once each (older children, three times each). Results go into a shared table on a large sheet of paper.

Minutes 63–70. Wrap-up. Look at the shared table: “What happens when the balloon is blown up more? Did it happen with every car? Which car went furthest and what's different about it?”. Read the children's version of “The science behind it”. Each child fills in their notebook: “My car went ___ m. What worked best was ___. Next time I'd change ___”. Compare with their first guess.

Minutes 70–75. Tidying up. Burst balloons in the bin (counted), cars on the shelf with names, trays clean.

If there's time left or another session: the challenge “the car that carries a load furthest” (stick on 1-cent coins: how many can it carry over 2 m?) or try a wider nozzle.

The engineering cycle

Engineers don't get it right first time: they follow a cycle that repeats. Teaching it explicitly helps children understand that failing and redoing is part of the work, not a failure. You can print the diagram and put it up in the workshop.

The project pages mostly cover the plan and create phases (because they give you the finished design). To work through the whole cycle, use the “Challenges for inventors” on each project page: they set a problem with no ready-made solution (a car that carries an egg without breaking it, a crane that lifts 200 g) and leave the ask, imagine and improve phases to the child.

Groups and classrooms

Doing a project with one child at the kitchen table and with 25 in a classroom are different activities. With a group, the challenge isn't technical but logistical: making sure everyone has materials, that nobody is waiting around with nothing to do, that the adult can handle the dangerous tasks without losing sight of the rest, and that the noise doesn't get out of hand. Each project page includes a “With a group or in class” section with specific guidance; this is the general picture.

Adult-to-child ratio

In a classroom with a single teacher, choose projects from the first row or prepare everything that needs adult tools in advance, and set up a hot glue station that the teams visit in turn. Ask for one or two volunteers (parents, older pupils) for the higher-risk sessions: an extra adult completely changes the session.

Teams and roles

For building projects, teams of 3 (ideal) or 4 (maximum). With 5 or more, someone is always just watching. For quick, cheap projects (thaumatrope, paper aeroplane), it's better to work individually. Give specific roles and rotate them every session, so everyone gets a turn at each one:

Laminated role cards or coloured pegs on clothing make it clear who is doing what. Remember that everyone builds: the role just adds a responsibility.

Stations

When there are tools that can't be on every table, organise the classroom into stations:

• Hot glue station: a table next to a socket, with 1–2 glue guns, a ceramic base, a glass of cold water nearby and an adult who stays put. Teams put their names on a list on the board and go when it's their turn.
• Cutting station: cutting mat, metal ruler, craft knife; adult only (or supervised over-11s).
• Testing station: the car track, the catapult target, the bowl of water. With a shared record sheet.
• Shared materials station: tape, glue, felt-tip pens, spares.

For short chemistry or light projects, another option is a carousel: 4–5 different experiments on 4–5 tables, 12–15 minutes at each, with the printed project page on the table. It works very well with the optical illusions and the air and water experiments.

Preparing materials for 25

• One tray per team with everything needed counted out, plus 10 % spare on the teacher's desk (balloons, straws, bottle caps, LEDs: something always fails).
• Pre-cut whatever slows things down. If cutting out the 8 cardboard pieces takes a team 20 minutes, cut them yourself beforehand (or in an earlier “parts factory” session). Class time is for assembling, testing and thinking.
• Test a complete prototype before the session, with the same materials the pupils will have.
• Print templates on 120–160 gsm paper if you're going to use them as patterns.
• Ask families for recycled materials 2–3 weeks in advance with a specific list: “4 matching 30 mm bottle caps, 2 toilet roll tubes, 1 cereal box”. Check that they arrive clean.
• Electrical components: one labelled envelope or zip-lock bag per team. At the end, check that everything comes back (especially magnets and coin cells).

Example for the Balloon-powered car, 25 pupils in 8 teams of 3 (one car per team plus a demonstration one): 9 cardboard rectangles of 16 × 9 cm, 36 bottle caps (+4), 18 wooden skewers (+4), 27 straws (+5), 18 balloons (+10), 18 rubber bands, 9 printed templates, 1 roll of tape for every 2 teams.

Time and noise

• A visible timer (on the interactive whiteboard or a large sand timer). Announce the halfway point and the last 5 minutes.
• An agreed quiet signal (hand up, a rhythmic clap, switching the light off for a second), practised at the start. Don't try to talk over the noise.
• Acceptable noise: a workshop is noisy. The test isn't silence but whether you can hear your teammate without shouting.
• Instructions before handing out materials: nobody listens with a tray in front of them. Explain first, then the materials manager collects the tray.
• Planned noisy moments: testing balloons, rockets and buzzers in the corridor or the playground, taking turns.

Early finishers and late finishers

Those who finish early (there's always someone):

• First, the full test with measurements: many have “finished” without testing.
• A challenge from the project page (“Challenges for inventors”) or an experiment from the “Experiment” section.
• Being a “technical assistant”: helping another team by asking questions, without touching their project.
• Documenting: drawing the project with its parts in the notebook, writing the instructions for a younger child.
• Decorating, once everything above is done.

Those who are slower:

• Keep spare pre-cut pieces to hand out if cutting is holding them up.
• Prioritise: make sure they get to test the project, even if they don't decorate it.
• Let them finish in another session or at home: store the project with their name on it.
• Check whether the problem is fine motor skills, understanding the step or how the team is organised; each one is solved differently.

Inclusion

Links to the curriculum

The projects fit the core knowledge of Spanish Primary Education (LOMLOE, Royal Decree 157/2022) and, in Infant Education (age 5), the area of Discovery and Exploration of the Environment. Some matches:

To plan around learning situations, a good format is a 3–4-session challenge: session 1, the problem and the basic project from the project page; session 2, experimenting with one variable and recording data; session 3, redesigning to meet a challenge; session 4, presentation and assessment with the rubric.

Assessing without marks

At home there's no need to assess anything. But in the classroom, or if you want to see how a child progresses over the months, it helps to observe with clear criteria. The aim isn't to give the car a mark, but to see what the child can do: follow instructions, solve problems, explain, collaborate and improve. A project that doesn't work can come with excellent learning, and a perfect one with very little (if the adult made it).

Four-level rubric

Use it by observing during the session and at the wrap-up. You don't need to assess all five criteria for every project: pick two per session. The level names are designed to be shared with the children.

Portfolio

The portfolio is the collection of evidence of the child's work over time. It shows progress better than any mark. It can be the inventor's notebook itself or a folder. What to include for each project:

• A photo of the finished project and, if possible, one of the first prototype and another of the improved version
• A short video (10–20 s) of the project working, with the child explaining what it does
• The notebook page: question, prediction, data table and conclusion
• A drawing with the parts labelled
• A sentence: “The hardest part was... and I solved it by...”
• The self-assessment or the adult's observation on the rubric

Every 6–8 projects, go through the portfolio with the child: “Look at your first car and your latest one. What can you do now that you couldn't before?”. It's one of the most valuable moments in the workshop.

Wrap-up questions

Pick two or three per session, not all of them. The answers tell you more than any test:

• What does your project do? Explain it to someone who hasn't seen it.
• Which part was the hardest? How did you solve it?
• What happened that you didn't expect?
• Why do you think it works? What if we take this piece off?
• What did you change after the first test? Did it get better?
• If you did it again, what would you do differently?
• Where have you seen something that works the same way in real life?
• What question do you still have? What would you like to try next time?
• Who in your team helped you, and how?

How to celebrate

• Exhibition or “invention fair”. At the end of a term or a block of projects, each child or team presents a project to families or another class: 2 minutes to explain what it does, how it works and what they improved. A card next to each project with the name, project number and an explanatory sentence written by the child. Visitors can leave questions on sticky notes.
• Live demonstrations: car race, catapult target contest, bridge load test. With measurements and a results table on display, and with prizes for the best improvement and the best explanation too, not just the best result.
• Certificate. The site generates a certificate with the child's name and the number of projects marked as done. Hand it over at a special moment (at the end of the first month, or at 10, 25, 50 projects), with a specific sentence about something they've learnt.
• Inventors' wall: photos of each project on a wall or a board, with the date. Watching the wall grow is a real motivation.
• Teaching someone else: have the child teach a brother or sister, a friend or a younger child how to make the project. It's the ultimate proof that they've understood it.

Avoid prizes for “the prettiest” or “the best” with no criteria: they discourage most children. If there are prizes, base them on a variety of criteria (longest distance, best improvement, best explanation, team that worked together best) so that different children can shine.

General problems and solutions

Each project page has its own “If something goes wrong” table. This one gathers the problems that come up in many projects. Before anything else, apply the general rule: describe exactly what is happening, change only one thing and test again.

Glossary

Terms that appear in this manual and on the project pages, explained simply. Many also appear in the “New words” section of each project, with a definition to read to the child.

Acid

A substance that, like vinegar or lemon, tastes sour, reacts with bicarbonate of soda to produce gas and turns red cabbage indicator pink.

Action and reaction

Newton's third law: if one body pushes another, it receives an equal push in the opposite direction. It explains rockets and the balloon car.

Amp (A)

Unit of electric current. The projects use milliamps (mA): 1 A = 1000 mA. An LED uses about 10–20 mA; a small motor, 150–500 mA.

Anode

Positive leg of an LED or a diode: the long one. It goes towards the + of the battery.

Average

The result of adding up several measurements and dividing by how many there are. Used to sum up 3 repeats of a test.

Axle

Rod that a wheel, gear or pulley turns on. In the projects, a 3 mm wooden skewer.

Base (chemistry)

A substance that is the opposite of an acid, such as bicarbonate of soda or soap. It turns red cabbage indicator green or blue.

Battery

A device that produces electricity from a chemical reaction. It has a positive (+) and a negative (−) terminal.

Battery holder

A holder that keeps the batteries in place and has output wires. The batteries inside are usually in series.

Bearing

A fixed part that an axle turns in with little friction. In the projects, a straw glued to the chassis.

Belt

A rubber band or string that joins two pulleys and passes the turning from one to the other.

Buzzer

Electronic sounder. An active buzzer beeps just by connecting it to a battery; a passive one needs an oscillating signal.

Cathode

Negative leg of an LED: the short one, on the flat side of the casing. It goes towards the − of the battery.

Centre of gravity

The point where all the weight of an object can be considered to be concentrated. The lower it is, the more stable the object.

Chemical reaction

A process in which some substances turn into different ones, often with gas, heat or a change of colour.

Circuit

A closed path that current flows round, from the + of the battery, through the components, to the −.

Clearance

Play or free space between two parts so that one can move relative to the other without rubbing.

Conductor

A material that lets electricity through: metals (copper, aluminium, steel), salty water, graphite, salty modelling dough.

Crystallisation

The formation of ordered solid crystals when a solution cools or its water evaporates.

DC motor

A motor that turns when it receives current from a battery. If you swap its wires, it turns the other way.

Density

How much mass there is in a given volume. Less dense things float on denser ones: oil on water, water on honey.

Elastic energy

Energy stored in a deformed object that tends to return to its shape: a stretched rubber band, a spring, a blown-up balloon.

Electric current

The amount of electricity that passes through a circuit each second. Measured in amps.

Electromagnet

A magnet that only works while current flows through a coiled wire (coil), usually wound round an iron nail.

Fair test

An experiment in which only one thing is changed and everything else is kept the same.

Flute (of cardboard)

The zigzag layer between the two faces of corrugated cardboard. It forms channels that decide which way the cardboard bends.

Friction

The force that resists sliding between two surfaces. It slows down axles, but lets wheels grip.

Fulcrum

The support point a lever turns on.

Gear

A toothed wheel that passes its turning on to another one it meshes with. Two meshed gears turn in opposite directions.

Gear ratio

How many turns one wheel (gear or pulley) makes for each turn of the other. Worked out by dividing the teeth or the diameters.

Hinge

A joint that lets two parts turn relative to each other like a door. In cardboard: tape, living (folded) or axle hinges.

Hydraulics

Using an enclosed liquid to transmit force and movement, as in the syringe arm.

Hypothesis

A reasoned prediction about what will happen in an experiment, written down before testing.

Indicator

A substance that changes colour depending on acidity, such as red cabbage juice.

Inertia

The tendency of an object to stay as it is: still if it's still, moving if it's moving.

Insulator

A material that doesn't let electricity through (plastic, paper, dry wood, rubber) or that is poor at conducting heat.

Kinetic energy

The energy an object has because it is moving. The faster and heavier it is, the more it has.

Laminate

To glue several layers of cardboard together, with the flutes crossed, to get a stiffer piece.

LED

Light-emitting diode. A small, very low-power lamp that only works one way round and needs a resistor.

Lever

A rigid bar that turns about a support point and lets you trade force for distance.

Magnet

An object that attracts iron and has two poles (north and south). Like poles repel; unlike poles attract.

Mountain fold

A fold that forms a roof or ridge. On the templates, a dash-dot line.

Multimeter

A device that measures the voltage, current, resistance and continuity of a circuit.

Nozzle

A tube that a jet of air or water comes out of to push a vehicle along, like the straw on the balloon car.

Open circuit

A circuit with a break somewhere (a loose wire, an open switch): the current doesn't flow.

Parallel

A way of connecting components in which each one has its own path between the + and the −. If one fails, the others keep going.

Polarity

The way round a component must be connected: which side goes to + and which to −.

Potential energy

Energy stored by being up high. When the object falls, it turns into movement.

Pressure

Force spread over a surface. In an enclosed liquid, it is transmitted equally in every direction.

Prototype

The first version of an invention, made to test and improve it.

Pulley

A wheel with a groove that a string or belt runs through. It changes the direction of the force or reduces it.

Refraction

The change of direction of light as it passes from one material to another, such as from air to water. It explains the water-drop magnifier and the rainbow.

Resistor

A component that limits the current. It protects LEDs. Measured in ohms (Ω).

Series

A way of connecting components one after another, in a single path. The voltages add up; if one fails, everything fails.

Set square

A tool for drawing and checking right angles. Also, a triangular reinforcement in a corner.

Short circuit

A direct connection between the + and − of a battery with nothing to limit the current. It heats the wires and battery and drains it.

Slot

A narrow cut in a piece for another to fit into. Its width should equal the thickness of the cardboard.

Solar panel

A device that turns light into electricity. It has polarity: red wire +, black −.

Solution

A uniform mixture of one substance (the solute, such as salt) in another (the solvent, such as water). Saturated: it can't take any more solute.

Split pin

A metal fastener with a head and two legs that open out. Used for pivoting joints and as a terminal in circuits.

Surface tension

The force that makes the surface of water behave like an elastic skin. It lets a paperclip float; soap breaks it.

Switch

A device that opens or closes a circuit at will.

Tab

A strip left on the edge of a piece so it can be folded and glued to another.

Template (1:1)

A drawing of a piece at its real size, for copying onto the cardboard. Printed at 100 %.

Valley fold

A fold that forms a V towards you. On the templates, a dashed line.

Variable

Anything that can change in an experiment: the one I change, the one I measure and the ones I keep the same.

Volt (V)

Unit of voltage. An AA battery gives 1.5 V; two in series, 3 V.

Voltage

The force with which the battery pushes the electricity. Measured in volts (V).