NPR's Science Friday recently featured "a science project for the recession." It's Oobleck - made by combining simple corn starch and water in a ratio of 2:1. The resulting gooey mixture is termed a complex, or "non-Newtonian" fluid. Prepare a small batch and you'll see why it seems to defy all physical laws. Gently handled, it behaves like a liquid. But apply a quick stress, like a quick finger prick, and it behaves like a solid.
Check out the video on NPR.
Non-Newtonian fluids are being designed for use in helmets and bulletproof vests.
More amazingly, people have prepared giant pools of the stuff, which you can run across.