Little Johnny's class is reviewing the alphabet. His teacher knows that he has an "advanced" vocabulary for his age, so she avoids calling on him. When the teacher asks for a word beginning with "A," Little Johnny raises his hand. The teacher anticipates he'll say, "ass" so she calls on Mary Lou, who says "apple."
This continues because the teacher knows that Little Johnny knows a cuss word for every letter of the alphabet. Then she gets to "R." She can't think of any cuss words that begin with R, so she calls on Johnny.
He exclaims, "R is for rats -- big f**king rats, with 12-inch c**ks!"