If one were to subtract the kinetic energies of all fundamental particles within hydrogen, as well as all of the potential energy of all bound states between these particles, how much mass (as a percentage of the total) would remain? Ignore the kinetic energy due to the hydrogen's speed. And while I'm at it, isn't it weird that fundamental particles can be converted into the motion of another particle? For instance, an electron can be annihilated with a positron, producing photons which can in turn be absorbed by the electrons in another atom, promoting them to a higher state. In effect, one electron has been turned into the motion of a different electron!