We're glad you asked.
A: Oh, you know, it's a real brain-twister. Essentially, it goes like this: you go back in time and kill your grandfather before he has kids. But then, you weren't born, so you never went back in time, which means your grandfather wasn't killed, which means you were born, which means... read more.
A: That's easy. You just bootstrap your way out of it, duh.
A: It's all about the predestined consequences, man.