Like all movies about time travel, Bill and Ted's Excellent adventure raises many questions about the possibility and paradox of time travel. The entire film is an example of the predestination paradox in which Bill and Ted are "pre-destined" to travel back in time in the future forming a self-consistent causal loop.
Spoiler alert: In the final moments of the film, the time travelers exploit the Novikov self-consistency principle in order to break themselves into jail they promise that the future version of themselves to travel back in time and steal their Dad's keys and place them in a predetermined location. In a subtle moment at the beginning of the film, you'll notice that the Dad is missing his keys. Thus, Bill and Ted were pre-destined to succeed. Dude!
Other Films about Time Travel
This film was followed by a sequel Bill and Ted's Bogus Journey (1991) in which Bill and Ted go to hell. There is also a video game entitled Bill and Ted's Excellent Video Game Adventure (1991).
