Two — one to change the bulb, and one to write a song about how good the old light bulb was.
or
Because the teacher said it was a piece of cake!
or
Just one, provided there is a programmer around to explain how to do it.
or
Hot, because you can catch cold.
or
Climb up a tree and act like a nut
or
Because pepper makes them sneeze
or
A comb.
or