If neutrons from the fission of uranium-235 continue to split other nuclei causing further fission, a chain reaction has occurred.
The number of nuclei which undergo fission multiplies rapidly.
In order for a chain reaction to take place, a minimum of one neutron from each fission must trigger further fission.
At the same time, the mass of fission material must exceed a certain minimal mass known as the critical mass. If the material is less than this value, too many neutrons escape without hitting nuclei, preventing a chain reaction from happening.
