This applet simulates what happens in a nuclear chain reaction. Each green dot you see here represents a nucleus that spontaneously fissions, if a neutron hits it. An example for such a nucleus would be 236U. Once a nucleus fissions, its fission products will be represented by two red dots. In addition, it releases 3 more neutrons.
You can get the chain reaction going. How? Just drop in a neutron by clicking with the mouse somewhere inside the black area.
If you want to stop the reaction temporarily to analyze it, just press the "s" button on your keyboard. To continue, press the "s" button again.
If you want to start all over, just press the "n" button, and you get a fresh pile of fishionable material just waiting to explode ...