The trick lies in the fact that life forms have no obvious boundary. The fact that two creatures are two creatures, and not just a single creature is something that is just endowed to them by us humans. If one of the creatures dies, does it really die, or does the combination of the two creatures just loses half of its brain cells? That second view is just as valid. Hence, in that second view, nothing really ever dies; we (i.e., the universe) just lose brain cells. And, of course, we gain them as well.