Bait brings the leprechaun to the trap. The trigger is the part that moves. The catch is what holds him once the trigger fires. A trap with great bait and no trigger is a snack. A trap with a trigger and no catch is a starting gun. Build in that order anyway: decide on the catch, add the trigger, then choose bait that sits on the trigger. For a shoebox, the catch is the box, the trigger is the propped stick or the card floor, and the bait is a coin on the moving part.

Look at what you have. A shoebox, cereal box or tissue box gives you a deep, dark catch. A mason jar or plastic bottle gives you a clear catch that children can see into. A paper cup or egg carton gives you a small catch for a very small leprechaun. The catch has to be heavy enough not to bounce off, tall enough that a toy cannot climb out, and open enough that the bait is visible. If the catch is a box, check that the lid is not sealed; cut a small air hole if it is.

The propped stick. Stand a craft stick on end under one edge of the box. Tie a string around the middle of the stick and run it out from under the box. The stick must be almost vertical, with the box resting on its top end. A tug pulls the stick sideways. Test it with a coin; if the box does not fall, the stick is too tight against the box or too thick. The trapdoor. Cut a square hole in the lid and tape the square back on one edge as a hinge. Rest the free edge on two straws taped under the lid. Bait goes on the free edge. Weight tips the card down. The hinge must be loose; tape only one side. The one-way flap. Cut a door in the box and tape two overlapping card flaps at the top inside. The flaps swing in but not out. Test with a toy; if it can push back out, the flaps are too short or taped too loosely. The jar prop. Lay a jar on its side over a paper plate. Prop the mouth open with a short stick. Tie a string to the stick and pull. The jar drops mouth-down. Put a towel under the jar so it does not crack.

You need a shoebox, two craft sticks, 3 feet of string, tape, a coin and a small toy. Turn the box upside down. Cut a 1-inch notch in the middle of the front rim. Tie the string to one stick. Stand that stick in the notch with its top under the box rim. Run the string under a chair leg 2 feet away so it pulls straight, and keep the end out of sight. Put a coin under the box, a hand's width from the open edge. Test with a second coin: drop it on the bait. The box should fall. If it does not, shorten the stick by a quarter inch and try again. When it works, add a sign and set it after bedtime.

One successful drop is luck; three is a mechanism. Drop a coin on the bait spot. Then pull the string from where you will actually sit. Then place a small toy on the bait. If all three work, the trap is ready. If one fails, change one thing only: the stick angle, the amount of tape on the hinge, or the position of the bait. Write the change on the plan sheet. That note is the beginning of a school project and the end of a frustrating evening.

Bait has two jobs: it has to be seen from outside, and it has to sit on the moving part. Put the shiniest piece where it shows, and the piece the leprechaun will grab on the trigger. A trail of three coins from the door to the trap tells a story and slows him down. Do not tape bait down. Do not use a pile so tall that it props the box open. If the catch is a jar, put the bait on the plate under the mouth, not inside the neck.

The box is too light. Tape a small book inside the lid. The stick binds. Sand the stick or widen the notch. The bait is too far in. Move it to the edge where the trigger is. The door swings both ways. Add flaps or tape the hinge so it only opens inward. The trap is on carpet. Move it to a hard, flat surface. The string snags. Run it under a chair leg and keep it straight. The catch is too shallow. Add a card collar or use a taller box. Fix one thing at a time, then test again. A trap that has been adjusted twice is more likely to work than a trap that has never failed.

Set the trap on a hard floor or table, away from pets. Run the string to your chair and keep it out of sight. Put the bait in last. Add a sign, a few footprints and a coin trail. Do not test the trap again after the child is asleep unless you can reset it exactly. Leave the note and a spare coin nearby so the morning has a payoff even if the trap is empty. If the trap fires during the night, leave it fired; the note can explain the escape.

A school project needs a design, a mechanism, a test and an improvement, not a perfect trap. Photograph the build, write down the materials, draw the trigger and record what happened in three tests. The free STEM worksheet has a K-2 and a 3-5 version with boxes for each part. A trap that failed twice and was fixed scores better than a parent-built showpiece. The school project guide breaks down the rubric row by row.