Joke: A rabbit

Teacher: If I gave you two rabbits, Billy gave you two rabbits, and Mary gave you two rabbits, how many rabbits would you end up with?
Tom: Seven.
Teacher: No, again, listen carefully this time, I gave you two rabbits, Billy gave you two rabbits, Mary gave you two rabbits, and how many rabbits did you have in the end?
Tom: Seven.
Teacher: That's still not true. Let me put it another way. If I give you two pigeons, Billy gives you two more pigeons, and Mary gives you two more pigeons, how many pigeons do you have in the end?
Tom: Six.
Teacher: Very good! Now let's go back to the question: If I gave you two rabbits, Billy gave you two rabbits, and Mary gave you two rabbits, how many rabbits would you end up with?
Tom: Seven.
Teacher: What's the matter with you? Aren't rabbits the same as pigeons?
Tom: No, I already have a rabbit at home.