He went on and on about how delicate space and time was, but frankly I didn't buy it.  I mean, if you think it's so easy to change the course of world events, try it.  You don't need a time machine.  You're already living in somebody's past and somebody else's future.  Just step on a bug or something and see what that gets you.  See if you were never born, or suddenly now there's fifty Hitlers in your bathroom, crapping all over everything.  It ain't going to happen.  Anyway, that's what I figured.

--The Time Machine Did It by John Swartzwelder