Wil's hit points

Wil and I have been playing some Battle for Wesnoth (as I mentioned before). Wednesday he asked me from the other room what 6+9 was. I told him and didn't think much about it. Then I heard him saying he had 15 hit points. I wondered where he came up with that number. He had put on his soccer shirt, which was #6, and added 9 (6 upside down, naturally). Fun...