Uranium and Dominoes
A video showing a domino chain reaction
You can't really do an experiment using uranium, because it is radioactive and if you experimented with uranium it would make you sick or kill you. So to see how a chain reaction works, let's play with dominoes instead of uranium atoms - that will be safer.
Get a set of dominoes and stand them up on end in a row. They should be about half an inch apart. Now tip the first one over so it hits the next one. They should all knock each other over, one by one. That's a chain reaction. Now watch these guys in the video knock over 4000 dominoes in a row! Cool.