None. The light bulb contains the seeds of its own revolution.
or
She was the fairest in the land.
or
A scale.
or
Two. One to change the bulb, and one to write a song about how good the old light bulb was.
or
Someone stepped on its mouse.
or
Two — one to screw it most of the way in and the other to give it a surprise twist at the end.
or