One; after reflecting in the twilight on the merit of the previous bulb.
or
A computer mouse.
or
Because you better duck when he's pissed.
or
Bugs Bunny.
or
They both look good hanging from a tree.
or
None, because they were copycats
or
To get to the udder side.
or
Because Mercury moved in.
or
Beef jerky
or
Because they don't want to give away their presence.
or