None, but it takes at least one to sit and pray for the old one to go back on.
or
One's a spineless, poisonous blob. The other is a form of sea life.
or
2 years old.
Cow: "Mooooove over"
Sheep: "Naaaaaaa."
or
She ran away from the ball.
or
One is a blood-sucking parasite, the other is an insect.
or