Fission reactions occur when the nucleus of a heavy atom, like uranium-235 or plutonium-239, splits into two smaller nuclei, releasing a significant amount of energy. This process is initiated by the absorption of a neutron, which makes the nucleus unstable. As it splits, it also releases more neutrons, which can trigger further fission reactions in nearby atoms, leading to a chain reaction. These reactions are the principle behind nuclear power plants, where controlled fission generates electricity. However, they can also be harnessed in nuclear weapons, where an uncontrolled chain reaction results in a massive explosion. Understanding fission is crucial for both energy production and nuclear safety.