I swear its in the book! LOL,i'm going to be re reading them soon so I deffinately will post it if i find it. Me and my friend were discussing this because in the end somewhere Harry finds the sorting hat again and he's like "why would i have been great in slytherin?" i'm almost positive its in book 6 or 7,most likely 7. I'm currently reading a diff series but maybe i'll try to thumb through or just read them and i'll post it when I find it. I remember discussing this with my friend last week and she agreed that it was in the books. Everything is explained in book 6 and 7.