It's been a long time since I've had to use this, and once I was out of school I never used it again. But I've been wanting to show my kids this trick just to see if they can confuse their teachers with it.
That is, how to subtract by adding.
I know that the likelihood of people who read this blog already knowing some computer math is quite high. In fact, I suspect many of you know a lot more than I do. But in case you don't: computers do not subtract. They only add. And here are some articles that explain how it's done: ones' complement, two's complement, and especially method of complements, which gives examples of how to do it in base 10.