How many science fiction writers does it take to change a lightbulb?
Two, but it's actually the same person doing it. He went back in
time and met himself in the doorway and then the first one sat
on the other one's shoulder so that they were able to reach it. Then a major time paradox occurred and the entire room,
lightbulb, changer and all was blown out of existence. They co-existed in a parallel universe, though.