The teacher was telling her 4th grade class about today's lesson.
"I'll say a letter of the alphabet and you give me a word that starts with that letter. Let's begin. A"
All the children raise their hands, but little Johnny was almost coming out of his seat trying to get picked. The teacher knew Johnny had a filthy mouth and thought to herself that if she picked Johnny, he would give her a word like 'ass' or 'asshole'. She picked Wendy, and Wendy said "apple".
"Very good", said the teacher, "now B".
Johnny was jumping out of his seat again, but the teacher picked Bobby. Bobby said "ball".
This went on and on with Johnny trying to get picked for each letter and the teacher knowing there was a dirty word for it. Then she got to "R". Nobody but Johnny had their hands up.
The teacher thought and thought and couldn't think of a bad word that started with "R". So she picked Johnny.
Johnny stands up and says: "R...Rat...a big, fat, fuckin' Rat!"