Two — one to change the bulb, and one to write a song about how good the old light bulb was.
or
The letter H.
or
2 years old.
Cow: "Mooooove over"
Sheep: "Naaaaaaa."
or
She couldn't control her pupils.
or
Five. One to screw it in and four to tell him how bitchin' he looked doing it.
or