Teacher: “If I gave you two cats and another two cats and another two, how many would you have?”
Little Johnny: “Seven.”
Teacher: “No, listen carefully … if I gave you two cats and another two cats and another two, how many would you have?”
Little Johnny: “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?”
Little Johnny: “Six.”
Teacher: “Good. Now if I gave you two cats and another two cats and another two, how many would you have?”
Little Johnny: “Seven!”
Teacher: “Johnny, where in the heck do you get seven from?”
Little Johnny: “Because I’ve already got a cat!”
Attribution: Fred