Cinderella is going to go out with Prince Charming. She approaches her Fairy Godmother and asks for some birth control.
"Absolutely not!" says the Fairy Godmother.
"Fine," says Cinderella. "But just remember this when there are all of these little princes running around."
So the Fairy Godmother thinks about it and relents. "Okay," she says, "I've turned this pumpkin into a diaphragm. However, you'd better be back before midnight, because that's when it will turn back into a pumpkin."
So Cinderella goes out, and the Fairy Godmother waiting ... and waiting ... it;s 12:00; then 1:00; then 2:00.
Finally Cinderella comes staggering in at 2:30, a dreamy look in her eyes.
"Where have you been?" demands the Fairy Godmother
"Out," says Cinderella.
"Didn't that diaphragm turn back into a pumpkin at midnight?"
"Yes."
"Well ... what did you do?" asks the Fairy Godmother.
So Cinderella says, "I met the nicest guy ... named Peter Peter something or other...."