How does a 6yo figure out negative numbers? I have no idea. Here's what happened during a lesson on how to use Singapore's bar models.
I was trying to show D-Man how the bar models correlate with addition and subtraction problems. In the course of the lesson, I used 6-10 as an example of a problem that doesn't work. Of course, that sent up a red flag with D-Man. He told me that it does have an answer and that he knew "all about negative numbers." Then he asked me to write a number line on the board and to add negative numbers to it. I did.
He then told me the answer was -4. After that I came up with a couple of similar problems and he solved them just fine. Then, he told me that he wanted to come up with his own problem. Did he come up with one like mine...no, of course not. He wrote down -20 + -10 =. Just as I was about to stop him and explain that negative numbers can be a bit tricky, he writes down -30. I stared at it dumbfounded. How did he figure that out? I have no idea, but he got it right.
So ends another math lesson at our house.