A friend showed me this earlier tonight, and I don't think it has been posted here before. So.. http://www.youtube.com/watch?v=QsTqspnvAaI
While the version available for download at http://www.kloonigames.com/blog/games/crayon only allows rectangles to be drawn, it's still a lot of fun. We were thinking it could work really well on the DS. I think it's a neat idea, surprisingly refreshing and intuitive. Anyway, give it a shot~ And after doing so, check out more videos like this to see some people who are really good with collisions.
XI Wiki

