It was Mark who first told me about Oobleck (non-Newtonian fluid made out of cornstarch and water). I’ve had a vague desire to play with it ever since (and since that was at least 20 years ago, it’s a pretty freakin’ vague desire. I think if I was that interested in playing with the stuff, I could have gooped up a batch of it by now).
However, there’s so much fun to be had with it. The Mythbusters played with it in lieu of Ninja water-walking, and YouTube is full of ooblecky videos. But BoingBoing showed me this one today, which I just love: a half-sheet pan of oobleck + a subwoofer = geekly joy.
http://www.youtube.com/watch?v=Px9jcA4decA