How we cite our quotes: All quotations are from Back to the Future.
Quote #1
Marty: Yeah, well history is gonna change.
But… can it, though? If it changes, was what you changed still history? Or is it a new history?
Quote #2
Marty: Where the hell are they?
Doc: The appropriate question is, "when the hell are they?"
Although we already knew going in that this was a time travel movie, this is the first time we're told about it point blank. A dog in a DeLorean vanishes, and it's not because he's been magically whisked away to Disneyland; rather, he's sent one minute into the future. And by the way, if PETA catches wind of this, Doc will wish he only had to deal with the Libyan nationalists.
Quote #3
Doc: He's fine, and he's completely unaware that anything happened. As far as he's concerned the trip was instantaneous. That's why Einstein's watch is exactly one minute behind mine. He skipped over that minute to instantly arrive at this moment in time.
Here's where we get the explanation, and it's setting the stage for what's to come. We're told a bit about how time travel works, but we get the information in the form of an exciting, one-minute canine journey, rather than just being lectured at in a chunk of exposition.