Little Johnny doing Math

The math teacher asks Little Johnny: “If I give you 3 cats, and then another 3 cats, and then again another 3 cats, how many cats would you have?” Little Johnny responds: “ten.” Teacher: “Ok… that’s not correct, let’s do this again. But pay attention this time. If I gave you 3 cats, and another 3 cats and then another 3 cats, how many cats would you have?” Little Johnny responds again: “Well… ten.” The teacher, becoming slightly frustrated, tries a different way: “let’s try another example. If I gave you 3 oranges, then another 3 oranges, and then again another 3 oranges, how many oranges would you have? Little Johnny: “Well, 9.” Teacher, happier, responds: “That’s correct. Now if I gave you 3 cats, and another 3 cats and another 3, how many cats would you have?” Johnny: “Ten!” Teacher: “Little Johnny, how did you even calculate 10?” Little Johnny: “Because I already have a cat at home!”