Splitting Atoms - How does it stop?

[SunOfBeach]

I'm going to talk like a 12yr old here, as I only have a 12yr olds understanding of this:

As I understand it, you shoot a neutron at an atom, and it causes the already unstable atom to get more unstable and hence split. It then releases other neutrons, which then can split other atoms in a chain reaction. So, 1 neutron leads to 2, 4, 8.... etc.

When/how does it 'stop', and cause an explosion? What causes this process to cease from going on forever?