Why is energy released when the nuclear reactions fusion and fission take place?

Energy is released during nuclear fusion and fission due to the conversion of mass into energy, as described by Einstein's equation E=mc². In fusion, lighter atomic nuclei combine to form a heavier nucleus, resulting in a mass deficit that is converted into energy. In fission, a heavy nucleus splits into lighter nuclei, also producing a mass deficit and releasing energy. Both processes occur because the products have a lower total mass than the reactants, leading to the release of energy.