Nuclear fusion is a nuclear reaction. It involves forming a heavier substance from two lighter ones. The reaction itself involves nuclei – the central part of an atom. Two lighter nuclei smash together and form a larger one, as shown in the animation to the right. The mass of this larger nucleus that is formed is not just the total mass of the two lighter ones, as some mass is transformed into energy. The two nuclei cannot always simply combine, and so there are other by-products of the reaction as shown in this animation.