CORRECTION: In the episode, Jesse mentions the movie The Manhattan Project II. This was in error. The film in question was actually called The Philadelphia Experiment II. Unfortunately, this error wasn't discovered until after the episode was published. The Media References section and this page have been updated.
Jesse & Jonathan define and discuss what's probably the most common time travel theory: Variable Thread.
Variable Thread: A time travel theory wherein a traveller is able to affect the timeline and change events.
Variable thread: Be sure to consider small changes to the timeline as well as the big ones, but only in service of the story. You can't map every single difference that expand out from your change, so pick your battles.
What, if anything, are the travelers trying to change?
What do they actually change?
How do your changes affect the thread?
A Sound of Thunder, Ray Bradbury
The Philadelphia Experiment II
Back to the Future II