Because it might crack up!
or
No one knows. Republicans automatically disbelieve him, and no one can ever trust a stinking liberal anyway.
or
None. There never was any light bulb.
Notes: Probably the only really good light bulb joke of 1984.
or
Super Pickle!
or
Silly, WASPs don't screw in a light bulb, they screw in a hot tub.
or