It left its tracks.
or
Two. One to change the bulb, and one to write a song about how good the old light bulb was.
or
None. The darkness will cause the bulb to change by itself.
or
Because he never pays his debts.
or
Because there are too many ears.
or
Two. One to screw it in, and one to stab the other in the back and take all the credit.
or