[He smiles wryly, at this.] Y-Yes, um, I...know the feeling, sad to say.

The Grandfather Paradox is simply the theory that, if you should happen to manage to go back in time and murder your grandfather so that all his descendents including yourself would cease to exist, you then could not actually have gone back in time and killed him, because, um, you w-wouldn't have existed to do so.

It requires a great many assumptions, but I suppose it gets the point across.