Transcription: In nuclear fission, a neutron bashes into a large nucleus, such as uranium-235, and splits it into two smaller nuclei, like this. Notice that the fission not only produces two smaller nuclei, it also produces a few neutrons. Those neutrons can go start fission reactions with other uranium nuclei, producing even more fissions, which produces even more neutrons. If this is allowed to eventually you have a chain reaction and a nuclear explosion.