Doctor: Time can be re-written
Amy: I thought you couldn't go back into your own time stream or anything, destroy the universe that sort of thing
Doctor: That what I thought before BUT....time can be re-written
Rory: So you're saying we can go back and change history? That doesn't seem right. I mean if it were true wouldn't someone have already done that? Like the second world war?
Amy: Yeah surely someone would've been all 'let's kill Hitler!'
Doctor: Yes! A great suggestion! Let's kill Hitler!