Light Bulb - Cops

Q: How many cops does it take to screw in a light bulb?
A: Just one, but he is never around when you need him.

Daleks to Change Light Bulb

Q: How many Daleks does it take to change a light bulb?
A: Daleks don't change light bulbs, they level the building.

Lightbulb - Aerobics

Q: How many aerobics instructors does it take to change a light bulb?
A: Five. Four to do it in perfect synchrony and one to stand there going "To the left, and to the left, and to the left, and to the left, and take it out, and put it down, and pick it up, and put it in, and to the right, and to the right, and to the right, and to the right..."