Two — one to screw it most of the way in and the other to give it a surprise twist at the end.
or
Because he want to wake up some day as America's First Dictator.
or
The hundred yard dash.
or
Because E.T. eventually went home!
or
One; she designs the bulb to crawl up the wall, unscrew the old one, and screw itself in.
or
Nearly unanswerable, since the one who tries to change it usually drops it, and the others call for a planning session.
or