Time travel is impossible everywhere except Hollywood, where it seems to come up at least a few times a year (just take the new Tom Cruise flick "Edge of Tomorrow"). In fact, it's so common in movies that "time travel movie" might as well be its own genre. And there's one thing these movies have in common other than defying the laws of physics: the characters love talking about the rules of time travel.
Every movie from "The Time Machine" to, well, "Hot Tub Time Machine" has had fun trying to explain to the audience just what the characters can and can't do. Can you change the past? If you're Bill and Ted or Marty McFly, sure! If you're Kyle Reese in "The Terminator," not so much.
The rules of movie time travel are pretty complicated, but you don't have to be Stephen Hawking to enjoy as your favorite movie characters explain them to you.
The biggest rule, though, might come from the aptly-named Basil Exposition from "Austin Powers": just sit back and enjoy yourself.