The Star Trek thread below mentions time travel and paradoxes. Years ago I had an idea of a rule for the universe to follow that would remove the possibility of paradoxes and "alternative timelines". I never discussed it because I thought I might do something creative with the idea. At this stage, that is no longer a possibility so here it is:

There is one and only one timeline and it is infinitely modifiable by going back or forward in time (should you have the means to travel in time). The kicker rule is that once a change is made, it propagates through the timeline only at the speed of time.

If you go forward (or backward) to time A, change something, go immediately forward to time A + 1 year, you will find that the change was never made. However, if you make the change, go back to your own time, wait a year and then travel to A + 1 year, then you will find that the change was made.