So I'm currently trying to wrap my head around nuclear fusion, but there is something that keeps bugging me: I've read in many places that when two hydrogen atoms combine, the resulting helium atom has a mass inferior to the sum of the mass of the two original atoms, because this mass has been given off as energy. But why? Why does some of the mass become energy? Is it something that we don't know yet? I've read about binding energy as well, but I cannot understand how it relates to energy being released during a fusion, or what it even is.