In class, Tommy’s learning subtraction. The teacher asks

"Tommy, if I have 5 cats and I give you 2, how many do you have?"

Tommy replies, "3."

The teacher sighs, "No, Tommy. I would have 3 left. 5 minus 2 is 3. You would have 2. Let’s try again, if I have 5 cats and give you 3, how many do you have?"

Tommy thinks for a second and says, "4."

Now the teacher's getting a bit confused. "No, Tommy… I gave you 3, so you have 3. Okay, last one, if I have 5 cats and give you all 5, how many do you have?"

Tommy, without hesitation: "6."

The teacher blinks. "Tommy, how do you have 6? I only gave you 5."

Tommy just shrugs: "I already had a cat at home."

Teacher: "No TOMMY. That’s exactly what I’ve been trying to tell you!"