Cardboard bends; LEGO stays where you put it. A gate built between two brick towers slides in a straight line. A plate floor with hinge tabs tips on a predictable edge. A baseplate gives the whole trap a stable footprint, which matters when a string is pulled. LEGO is also forgiving: if a groove is too tight, rebuild it; if a gate is too light, add a plate. The trap can be taken apart and rebuilt after the test, which is the whole point of a design project.

Build four walls 6 bricks high with a 4-stud doorway. Build a gate from a 4x6 plate with two side rails and slide it into grooves made by two towers. Add a lever inside: a 2x4 plate on a 1x2 brick pivot, with one end under the gate. Tie a string to the other end and run it out between bricks. Pull the string, the lever lifts and the gate drops. Test with a toy before you put the bait inside.

Build a 6x6-stud tower 8 bricks high with an open side at the bottom. Make a 4x4 plate floor with two 1x2 hinge tabs and clip them over a 1x4 beam across the top. Put a coin on the unsupported edge. The floor tips when weight lands on it. Add a sloped tile shelf inside so the catch slides toward the open side. This build takes longer but gives the child a mechanism to explain.

Build a low pen 4 bricks high with a 4-stud opening. Build a ramp from smooth tiles and plates, sloping from a stack of bricks down to the opening. Tape card rails along the edges. Tape two card flaps at the top of the opening inside the pen. Roll a marble down to test. It should push through the flaps and stay in the pen. Adjust the ramp height until the marble arrives with enough speed.

Build a 6x6-stud box 6 bricks high with a solid floor. Build an 8x8 plate lid with a 1x4 beam handle and hinge it to the back wall with clips or tape. Prop the lid open with a LEGO beam. Tie a string to the beam. Put a coin inside and pull the string: the beam slides out and the lid drops. Add a weight plate if the lid does not close.

The gate must be heavier than the friction in its grooves. Test it without the string first: lift the gate, let go, and watch it drop. If it sticks, move the guide towers one stud apart. If it falls too fast and bounces, add a plate to the bottom of the gate. The string should pull straight, not at an angle. Run the string through a gap between bricks, not over a sharp edge. Test three times.

Use wooden blocks, DUPLO, magnetic tiles or a mix. The mechanism matters more than the brand. A cardboard gate in a cardboard groove works the same way. If the blocks are too light, tape a coin inside the gate. If the stack falls apart, set it on a tray and lift the tray into place. The plan sheet has a sketch box for whatever material you use.