Three:
One to write the light bulb removal program,
one to write the light bulb insertion program, and
one to act as a light bulb administrator to make sure nobody else tries to change the light bulb at the same time.
or
On honor rolls.
or
the barman says "Oi - get out! We don't want your type in here"
or
She ran away from the ball.
or
They charged one - and let the other one off.
or
A centipede with a wooden leg.
or