Seven cats

Teacher: “Billy, if I gave you 2 cats and another 2 cats and another 2, how many cats would you have?”

Billy: “Seven.”

Teacher: “No, let’s try again… you get another two cats, and another two cats and another two, how many would you have?”

Billy: “Seven.”

Teacher: “Let me put it to you differently. If I gave you two apples, and another two apples and another two, how many would you have?”

Billy: “Six.”

Teacher: “Good. Now if I gave you two cats, and another two cats and another two, how many cats would you have?”

Billy: “Seven!”

Teacher: “Billy, where you get seven from?!”

Billy: “Because I’ve already got a cat!”