It'll come back to me.
or
Three, One to climb the ladder. One to shake it. And one to sue the ladder company.
or
1,000,001: One to change the bulb and 1,000,000 to rebuild civilization to the point where they need light bulbs again.
or
Five. One to screw it in and four to tell him how bitchin' he looked doing it.
or
Because they don't fit on a ironing board
or