A sponge!
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
Just one, provided there is a programmer around to explain how to do it.
or
"The way you make me feel, it really turns me on!"
or
proving once and for all that you can't have your kayak and heat it.
or
An impasta
or