The teacher called Little Johnny to her desk. She said: “This essay you’ve written about your pet dog is exactly the same essay your brother has written.”
“Of course it is,” said Johnny. “It’s the same dog.”
Little Johnny's teacher is surprised that he and his brother have the same essay. Johnny casually points out that they have the same dog, implying that the essay was about the dog rather than a separate personal narrative from each student. This is funny because it's an absurd way to get out of trouble and implies that essays are just about describing a subject rather than applying personal thought.