I prefer 'Clack'

Seems like a good explanation. The one difference with other explanations I've heard is that it's usually said that Sephiroth's real body wasn't destroyed/killed - just that it entered the lifestream (same way as Cloud did) when he got thrown from the reactor, and then got carried by the lifestream to the North cave where he ended up in that blob of mako/materia for a while controlling Jenova's body and making it appear as himself. Essentially, by this explanation, there were 2 "Sephiroth" bodies in existence, one his real body and mind controlling the 'fake' one which is Jenova's body (puppet).