Play this game now. It's incredible; you draw two dimensional objects in-game, and they interact with everything else, in a way that fulfils a childhood fantasy I'm sure everyone had. I played through the full thing about five times in a row, and I'm still not bored of it.
http://www.kloonigames.com/blog/games/crayon/
You'll love it.
Best thing is, he's making an updated version - and if you look at the video for it, it looks even more amazing that the original.
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Very fun game, never had so much fun with physics :D