SirKirbance
MyOwnBestCritic
Then there's the grandfather paradox. Let's say you go back in time and kill your grandfather when he's still a baby. You wouldn't exist, right? So, then, who killed your grandfather? [/mindboggle]
There is also the Back to the Future 2 theory, that says that when you time travel it actually involves paraellel universe. So if you travel back in time and kill your grandfather, you really killed him in another dimension. But he is not killed in your dimension. But I don't really think it works that way.

That would be a great explanation if we could prove the whole parallel universes things...