In order to explain a nuclear fission reaction, the folks at Harvard University set up a grid of mousetraps and ping pong balls.
In a nuclear reactor or atom bomb, a fissile material such as 235U can capture a neutron. The resulting unstable nucleus fragments into two smaller nuclei, releasing energy and several neutrons (a typical equation is given below). Each of these neutrons can in turn cause the fission of a 235U nucleus. If there is above a critical concentration of fissile material, this chain reaction will continue unaided, and if unregulated, can result in a very loud bang.
We have a 120 × 70 × 100cm high plexiglass case, onto whose base we set a 5 × 20 array of mouse traps. 1 Onto each trap is rested a ping-pong ball. The traps represent the fissile atoms, and the balls the neutrons. When an extra ping-pong ball is dropped through a hole in the top of the case, it lands on an triggers a trap. Now there are two ping-pong balls each capable of setting off a trap. Thus a chain reaction ensues; the whole explosion lasts about three seconds.
Also read an overview of how this contraption was built, in case you want to reproduce this effect yourself. Link -via Arbroath