I've been reading up on radiation and it appears to me that if you add up all of the protons, neutrons, and electrons in the universe, the number would never change (except for temporary positron borrowing). I guess this is no surprise considering conservation of matter. On the other hand, because E=mc2, I thought that matter was converted to energy - I see now that m has nothing to do with matter and everything to do with inertial mass. So I am hoping someone can help me out of my confusion - how is it that subatomic particles (at rest) change their mass? does it simply have to do with the strong nuclear force? Maybe my question should be what is mass?