Uranium 235 splitting for fission:

Uranium-235 has an interesting property that makes it useful for nuclear power production. U-235 decays naturally, just as U-238 does, by alpha radiation. U-235 also undergoes spontaneous fission a small percentage of the time. However, U-235 is one of the few materials that can undergo induced fission. If a free neutron runs into a U-235 nucleus, the nucleus will absorb the neutron without hesitation, become unstable and split immediately.