None. They will never find one that burned as brightly as the first one.
or
None. The darkness will cause the bulb to change by itself.
or
They get toad.
or
Saint Nickel-less.
or
or
Nothing, peanuts don't talk.
or
The neutron says "Are you sure?" The proton replies "I'm positive."
or
Executive Producers don't screw in a light bulb, they screw in a hot tub.
or
They both have a hole in one!
or
but unfortunately she'd popped her clogs.
or