For a few isotopes (an atom with the same amount of protons but a different amount of neutrons) such fission can release many neutrons. If those neutrons then hit other atoms, they will make the other atoms split. This can happen again and again. This is called a nuclear chain-reaction, and it can release huge amounts of energy very quickly. The amount of energy released by a nuclear chain reaction is measured in kilotons. One kiloton is the same as the energy of one thousand tons of TNT (trinitrotoluene).