Weather Vane

Teacher asks her class, “What is the animal depicted on a weather vane?”
Little Johnny, the infamous troublemaker, says, “I know, teacher, I know.”
Against her better judgment she calls on him and he says, “Teacher, it is a cock.”
Teacher asks the class, “Why is a cock on a weather vane?”
“I know, I know, teacher,” says Johnny.
“OK, Johnny, Why?”
“Because, teacher, if it had a cunt on it, the wind would whistle right through it.”