When deuterium and tritium nuclei collide and stick, the leftovers, (a helium nucleus and a neutron) are slightly lighter than the original nuclei. There are still the same number of components, 2 protons and 3 neutrons, but in the processing of rearranging themselves, a small amount of their mass gets turned into a large amount of energy.

The amount of energy released varies depending on the exact elements that are fusing – scientists have deduced that the most efficient reaction is that between deuterium and tritium, which is why it is the fuel of choice for fusion reactors.