I saw this on a blackboard in a CS class and improved upon it.
Q: "How many computer scientists does it take to screw in a light bulb?"
A: "Five. Two write the specifications, one to prove their validity and two to implement it."
Q: "Well, how many hackers does it take?"
A: "One. But, hackers don't turn on the lights."
[Ed: The master list of bulb jokes (hundreds) appeared in Volume 1 of this series.]