Light Bulb - Dan Quayles

Q: How many Dan Quayles does it take to screw in a light bulb?
A: One, but it has to be a pretty dim bulb.

Light Bulb - Kennedy Assassination Theorists

Q: How many Kennedy assassination conspiracy theorists does it take to screw in a light bulb?
A: 15--One to screw it in, five to say he acted alone, one to say that someone hidden in the ceiling helped, one to film it, one to do an intense examination of the film and conclude that a) it was tampered with and b) it proves that the first screwer did not act alone, one to insist that the bulb was altered after it was unscrewed, three tramps to walk across the room an hour later, one to insist LBJ really screwed the bulb in, and one to accuse all the others of being disinformation specialists.

Pessimistic

Q: How many pessimists does it take to screw in a light bulb?
A: None, the old one is probably screwed in too tight.