The teacher asked the class to name things that are found in a
barnyard.  Suzie raised his hand and said, "A weather vane."

Very good Suzie," said the teacher. She then asked, "Billy, do
you know what's on a weather vane?"

A cock," said Billy.  "Very good Billy," responded the teacher.
She then asked, "Johnny, why is there a cock on a weather vane?"
Little Johnny replied, "Because if there was a cunt on there
instead, the wind would whistle right through it."