Fission is induced when neutrons collide and are absorbed by nuclei. When these nuclei break apart, they release more neutrons which can induce fission in other nuclei. This is known as a chain reaction which is seen in Figure 1.

As each fission interaction releases multiple neutrons, this chain reaction can quickly grow, exponentially releasing more energy as it continues.

Figure 1: The fission of one nucleus can start a chain reaction of fission in other nuclei.