2+2+2 = 7

Teacher: If I gave you 2 cats and another 2 cats and another 2, how many will you have?

Johnny: Seven, Sir.

Teacher: No, listen carefully. If I gave you 2 cats, and another 2 cats and another 2, how many will you have?

Johnny: Seven

Teacher: Let me put it to you differently. If I gave you 2 apples, and another 2 apples and another 2, how many will you have?

Johnny: Six.

Teacher: Good. Now if I gave you 2 cats, and another 2 cats and another 2,
how many will you have?

Johnny: Seven!!!

An angry Teacher: Where the in hell do you get seven from ?

Very angry Johnny: Because I've already got a cat at home!!!