A few months ago, I watched in awe at the video demonstration of Crayon Physics Deluxe, but couldn't experience it myself - PC only, really needed tablet, only available version is an earlier version. Which is why I'm so glad to see the appearance of Magic Pen, a free online flash game that seems to bring a lot of the same ideas to a web browser. I played this WAY more than I should yesterday - there's something very addictive about drawing little seesaws and trebuchets.
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This is HIGHLY addictive. Every round is like a little IQ test.