Negative numbers

At night, Leo and I cuddle in bed and talk about the day and then he goes to sleep. Tonight he told me that he knew what one minus six was. I said, "What?" He said, "negative five". I thought maybe someone had told him that so I said, "That's pretty good. What's one minus two?" He said, "negative one". I told him that was pretty cool that he could figure out negative numbers. He said he didn't really understand them and were they like a mirror? Yep, I said, just like a mirror.