Two printed banks that slide anywhere from 6 to 40 cm apart, a box of connectors and skewers you can build any bridge from, and a ramp that always sends the marble off at the same speed.
Will it make it across?What’s new: a building kit where the beam lengths aren’t manufactured but cut from skewers, so the possibilities really are endless. A marble rolling off a ramp at a constant speed gives every build the same honest test.
The slider moves the banks apart in 2 cm steps, and for every span the code works out a reference truss bridge (a Warren truss): how many connectors, how many skewer pieces and how long they are. A challenge card passes only if at least one reference solution fits its parts list. The marble runs in 4× slow motion. Sound plays only after you click.
Free build with the whole box. Slide the banks apart and let the marble go.
My 9-year-old draws a card: “Span 24 cm, 14 skewers max, the marble has to reach the finish.” He slides the bank to the 24 mark and clicks the peg in. He builds two triangle trusses, lays the deck across them and lets the marble go from the ramp. It rolls off, crosses over and lands in the finish with a ding.
Every card changes the span, the parts list or the type of bridge (truss, arch, suspension, cantilever, tipping), so the same parts make a different build every time. Records like the longest span from twenty skewers or the heaviest load go on the family leaderboard.
The banks, a three-piece rail with holes every 2 cm, connectors with skewer sockets at fixed angles, deck pieces with a groove for the marble, the ramp, the finish, a cup for load tests and a tipping piece with a print-in-place hinge.
The beams aren’t printed at all: they’re plain bamboo skewers.
Claude writes challenge cards for a chosen span, parts list and difficulty, with pictograms for younger kids. From a photo of a failed attempt it explains why the bridge gave way (“the bottom chord had no diagonals, so it sagged”). On request it designs its own bridge from the same parts.
None in the first playable prototype.
The full version adds two IR light gates on an ESP32-C3 with a display for the crossing time, and maybe a 5 kg load cell under the finish bank.
Two banks, a three-piece rail, the ramp, the finish, 30 connectors, 8 deck pieces and the cup. Add a pack of skewers and 30 glass marbles. It plays without a Mac or a network, and the cards are printed in advance.
| Item | Price source | CZK |
|---|---|---|
| Filament ~525 g (banks, rail, connectors, deck pieces, ramp, finish, cup); 25 g more than the toy card lists, for the third rail piece | card | ~262 |
| Bamboo skewers 3 mm × 20 cm, 100 pcs | estimate | ~40 |
| Glass marbles 16 mm, 30 pcs | estimate | ~70 |
| String for the suspension bridge | from home | 0 |
| Total (rounded ~370 CZK, about €15) | ~372 | |
Custom connectors: code generates a connector from a template with whatever angle the challenge needs. Two IR light gates on an ESP32-C3 with a display for the crossing time (~250 CZK, about €10). If you want to measure load in grams, add a 5 kg load cell under the finish bank.
| Part | Dimensions and purpose |
|---|---|
| Rail | three 200 × 30 × 12 mm pieces joined with dovetails into 600 mm (each piece fits a 250 mm print bed), Ø 6 mm holes every 20 mm and a printed scale; the bank locks in with a printed Ø 5.8 mm peg; span 6–40 cm in 2 cm steps |
| Bank | a block 100 mm along the rail × 120 mm across × 80 mm tall (hollow, 15% infill), a grid of anchor sockets on the front and top at 0, 20 and 40 mm below deck level, an eyelet at the back for the suspension cable; bank A carries the ramp seat, bank B the finish |
| Connectors | flat ~8 mm hubs with Ø 3.3 mm sockets, 12 mm deep, for a Ø 3 mm skewer; types: straight 180°, L 90°, V 60° and 45°, T, K (0°/60°/120°/180°) and a cross connector that joins two trusses into a 50 mm wide bridge; printed flat |
| Deck piece | a 50 × 60 mm plate with a U-groove for the 16 mm marble, two hooks underneath for cross skewers, beveled ends |
| Ramp | a ~300 mm chute on a stand with notches for heights of 10, 15 and 20 cm, and a stop with a flip lever, so the marble always starts at rest from the same point |
| Tipping piece | a deck piece on a print-in-place hinge with a counterweight; the arms are set so a 16 mm marble outweighs it |
| Cup | ~40 ml with a hook for a cross skewer at midspan (see the jury below) |
A 40 ml cup holds only about 9–11 of the 16 mm marbles (~50–60 g), and the set has 30. A skewer truss can hold more than that, so the cup fills up before the bridge gives way. Instead of marbles, pour rice or sand into a hanging bag, and once the bridge breaks, weigh the bag on a kitchen scale.
Jury, round 5A Ø 3.3 mm socket for a Ø 3 mm skewer leaves only 0.15 mm of play, and nobody has checked that friction really holds. Skewers vary in diameter and printed holes come out smaller. Before printing 30 connectors, print a test comb with 3.0 / 3.1 / 3.2 / 3.3 / 3.4 mm holes and try it with skewers from the actual pack.
Jury, round 5For a 6-year-old, the 16 mm marbles and the connectors are small parts. Our daughter builds only with supervision, and the marbles and connectors live in a box with a lid.
Card and juryBuy skewers with blunt ends, or snip the points off right away. The first prototype has no press and no screw, so nothing gets overloaded and there’s nowhere to pinch a finger.
CardThe formula v = √(10/7 · g · h) holds for rolling on a flat surface. In a U-groove the marble rolls on a smaller radius, so it goes slower. That’s why the speed gets measured once with a stopwatch over one meter of track, and the code then works with the measured value.
Card and juryStraw Works, which I turned down, used the same idea of printed connectors and store-bought sticks. Bridgeworks, the one I picked, stands apart with its bridges, the ramp and the marble.
Jury, round 5“An open-ended building kit with an honest test (a marble off the ramp) and nine kinds of challenges. The best version of this idea so far; only the load challenge doesn’t work at the current scale.”Jury, round 5 · no STOP flag, watch the small parts around the 6-year-old
“The score goes up from 20 to 21. You can play the kit again and again. The press and the load cell are gone, so overloading and pinched fingers are off the table.”Jury, round 5, compared with round 4
One of eighteen finalist toy ideas in the Toy Workshop, generated by AI agents, picked by a dad.
Made by Ondra Malacka with Claude agents · malacka.cz