A simple nuclear chain reaction demo

Materials you will need:
A large clear plastic or plexi-glass box with a lid and a flat bottom
Cut a hole large enough for ping pong ball to fit through easily in the top of the box
Enough mouse traps to fill the bottom of the box (I used 18-20, but the demo shown here is on a larger scale and uses 85-100 traps)
A ping pong ball for each mouse trap, plus a few extras

Critical System:
To demonstrate a nuclear chain reaction at critical mass, set all the mouse traps close together at the bottom of the box. Place a ping pong ball on the metal set bar of the trap. If you can find a way to get them to stick, bottle caps make great ping pong ball holders. The balls will generally balance fine on their own, but the caps are nice to use. I tried crazy glue and 5 minute epoxy with no luck.
Throw a ping pong ball into the hole at the top of the box. This sets off one mouse trap, which flings its ping pong ball and will thus set off another, etc. Each mousetrap is supposed to represent an atom of nuclear fuel (such as Uranium 235) and each ping pong ball a neutron that will be made available after a fission event to fission other atoms.

Sub-critical system:
To demonstrate a sub-critical system, place a few mouse traps set with ping pong balls placed far apart in the box, including one set directly beneath your hole so that at least one trap will be set off. It will take some time experimenting to figure out what the 'critical mass for mousetraps' is. This a fun experiment to do with students, try and get them to make some guesses. For my system, critical mass turned out to be around 7-8 mousetraps.

Fissionable vs. Fissile fuel:
This is possible using only some varieties of mousetraps. Some mousetraps have 2 settings, a 'hard' setting, which requires more force to set off the trap, and a 'light' setting, requiring less force. This can be used to demonstrate the difference between fissile and fissionable nuclear fuels. Fissionable fuels can have fission induced by only fast moving neutrons. Fissile fuels can have fission induced by both fast neutrons and slow neutrons.
A 'hard' setting can be used to demonstrate fissionable fuels. You may either need to find an accurate way of speeding up the ping pong ball (ie. not just dropping it into the box), or you can use something heavier to set off the traps initially, like a golf ball.
A 'light' setting will set off the traps much more easily. Dropping the ping pong ball in at a relatively low speed will be enough to set off the chain reaction.

Caution: This can be a somewhat dangerous demo to set up. Some mousetraps are rather sensitive and can snap on your fingers. Wearing some gloves to protect your skin isn't a bad idea. Set the traps carefully. Children should not try to do this demo themselves.