How exactly does energy "become" mass? In Einstein's equation E=mc2 or more sepcifically the "inverse", m=E/c2, how exactly does mass "become" energy? I [vaguely] understand how energy may be released from mass, but how would mass come about from just energy? Maybe this question is unanswerable, or maybe I'm not asking it correctly. If its unanswerable in a few paragraphs, any suggestion on books that may be relevant to my question would help too.