My kid is a smart*$$ and that is sometimes a good thing

Galen has math homework each night. 3-5 division problems, one number divided into a 3 digit number. He was doing a counting up approach he learned in class, but was not doing true long division with remainders. As my division questions have got harder and harder, I just couldn't watch the counting up anymore. Let me show you another way, my son. So, we sat down and started to work through the long division steps.
How many times does 3 go into 1? none, OK include the next digit
How many times does 3 go into 10? 3 with one remainder; bring down the next digit.
How many times does 3 go into 12? 4. Great, so 102 divided by 3 is 34.

Trouble is, the guy wants to just do it in his head. He wasn't bothering to write down HOW he got his answer, which the teachers don't like because they assess the answer and the route to the answer. But G has to argue that its fine, no problem, because he can do it. And showed me over and over and over. So, I changed tactics. I'll show you that it is easy to make mistakes if you are trying to carry remainders in your head. Let me prove to you that writing it down is better. I'll give you a life lesson - I start increasing the number of digits. From 3 to 5, Ahh yes that is the correct answer. From 5 to 6, then 7, then 8 digits. Dude got them all correct, doing the math in his head. He laughed and the lesson was lost. Fine, you don't need to write it down because it is easy to mess up, you can keep track of remainders in your head. Smart*$$ son.

So my argument is back to "because your teacher told you to". Not the best reason but still a reason. He is taking in his work sheet to show his teacher and I am moving on to something more challenging. Clearly, math is his strength.