What is nuclear fission and how does it work?

Nuclear fission is the process by which a heavy atomic nucleus splits into two or more lighter nuclei, releasing a significant amount of energy.

This process is typically initiated when a heavy nucleus, like Uranium-235, absorbs a neutron, becoming unstable. The unstable nucleus then splits, producing two smaller nuclei (fission products), two or three additional neutrons, and gamma rays. The newly released neutrons can then go on to strike other heavy nuclei, causing further fission events and leading to a nuclear chain reaction. This chain reaction is the basis for nuclear power generation and atomic weapons.

The total mass of the products is slightly less than the original mass of the heavy nucleus and the initiating neutron. This mass defect is converted into a tremendous amount of energy according to Einstein's mass-energy equivalence principle, $E=mc^2$.

The final answer is $\boxed{\text{Nuclear fission is the process where a heavy atomic nucleus splits into two or more lighter nuclei, releasing a large amount of energy and neutrons.}}$