What causes nuclear fission?
Nuclear fission either happens naturally or can be caused to happen by bombarding a fissionable isotope with neutrons.
Nuclear fission happens when an atom splits into two atoms and releases energy.There are several naturally occurring isotopes which will split spontaneously and release energy, such as Uranium 235 and 238. These isotopes are called fissile.
Fission can also be forced to happen by bombarding a large isotope with a small particle, commonly a neutron, like in this animation: