Little Billy and addition

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

Little Billy: Seven Sir.

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

Little Billy: Seven.

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

Little Billy: Six.

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

Little Billy: Seven!!!

Teacher: WHERE DO YOU GET SEVEN FROM?!

Little Billy: Because I already fucking have a rabbit at home!