How does a Nuclear Reactor Work?

Nuclear reactors use rods of uranium to boil water, the steam turns turbines that produce electricity. The reason this works is that when the uranium atom decays it gives off neutrons which collide with other uranium atoms causing them to split in turn and the reaction goes on and on building up a massive amount of heat. If left unchecked the uranium rods will get hot enough to burn through the crust of the planet. Graphite absorbs neutrons and is therefore used to control the reaction.