The barman says "I'll serve you, but don't start anything"
or
Bike helmets don't fit hippos
or
None. Bill Gates will just redefine Darkness® as the industry standard.
or
2 years old.
Cow: "Mooooove over"
Sheep: "Naaaaaaa."
or
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
To prevent clients from being billed twice for essentially the same service.
or
Climb up a tree and act like a nut
or