Compare ""A Sound of Thunder"" and ""By His Bootstraps"" what common time travel rule do they share?

Not to alter the past.

Explanation:

The common time travel rule that "A Sound of Thunder"" and "By His Bootstraps" is not to alter or change the past. Both stories had the same rule as any slight change in the past or even killing a mouse, as in "By His Bootstraps", would have a negative effect on the future. Though history may not completely change by a time traveler's actions, there are chances that they might disrupt the events of things in the future.