Final answer:
In a nuclear reaction, atoms hit other atoms and cause a chain reaction. In the case of U-235 fission, when an atom absorbs a neutron, it can split into two new atoms and release neutrons. These neutrons can then hit other atoms and continue the chain reaction.
Step-by-step explanation:
In a nuclear reaction, the chain reaction occurs when one atom hits another atom and causes it to undergo fission. This fission releases multiple high-energy neutrons that can go on to hit and split more nuclei, continuing the chain reaction. In the case of U-235 fission, for example, when a U-235 nucleus absorbs a neutron, it can split into two new atoms, releasing three neutrons. One of those neutrons may be absorbed by U-238 or escape, but if one neutron collides with another U-235 atom, it can initiate fission and release more neutrons, continuing the chain reaction.