How many science fiction writers does it take to screw in a light bulb?
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 shoulders so that they were able to reach it.
However, a major time paradox occurred and the entire room, light bulb, changer and all was blown out of existence.
Moral of this joke: don’t screw around with time.
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