Catapult from popsicle sticks

Note: I switched from tin foil ammunition to foam cubes, to avoid eye injury. (The ammo tends to go straight upwards towards the students face unless they lean back).

How to make the catapult before class:
Use a saw to make a notch on either side of both jumbo popsicle sticks, about 2cm from one end.
Stack up 8 regular popsicles sticks. Insert the notched-end of one jumbo popsicle stick between the bottom two of the stack. Wrap the stack of 8 tightly with elastic bands on each side.
Lay the second jumbo popsicle stick over the top of the stack, with its notched end aligned with the jumbo stick tied into the stack. Wrap an elastic band around both jumbo sticks, making sure that it rests in the notches.

Make foam cubes to fire with the catapult. They will go about 1m into the air.
Make modelling clay cubes of the same size for students to compare with the foam cubes. They will not go nearly as high.

Discuss the pushes and pulls: push down on the catapult, then as you let go it pushes the ammo into the air. The ammo falls back down as gravity pulls on it.
Ask students how high the foam will go with less push [less high]: with the same object the amount of push determines how far it will go.
The heavier clay is the same shape and size as the foam, but it does not go as far: the same force from the catapult arm will push a heavier object less far.
Students will want you to try firing other objects. Note: be careful with small dense objetcs as they will go quite high (and fast), so make sure the trajectory is well away from the students.