This is a video of a non-Newtonian fluid on a metal sheet set atop a subwoofer. It's pretty cool to watch. In case you forgot (or never knew) what non-Newtonian fluids are, I've copy/pasted some Wikipedia below, and posted another video of some people running across a pool of the stuff after the jump.
A non-Newtonian fluid is a fluid whose flow properties cannot be described by a single constant viscosity. An inexpensive, non-toxic example of a non-Newtonian fluid is a solution of corn starch (corn flour) and water, sometimes called oobleck. The application of force - for example by stabbing the surface with a finger, or rapidly inverting the container holding it - leads to the fluid behaving like a solid rather than a liquid. This is the "shear thickening" property of this non-Newtonian fluid. More gentle treatment, such as slowly inserting a spoon, will leave it in its liquid state. Trying to jerk the spoon back out again, however, will trigger the return of the temporary solid state. A person moving quickly and applying sufficient force with their feet can literally walk across such a liquid.
Mix cornstarch to water in a 2:1 ratio to make your own. But if you don't have any cornstarch handy you can just use pancake batter.
UPDATE: Okay, who believed me and used pancake batter? Anybody, anybody? Suckers! This is the interwebs, folks -- you can't go around believing everything you read. And on an unrelated note, does anybody know how long it typically takes for a Nigerian prince to transfer funds into a U.S. bank account?
Hit the jump for a video of people running across a non-Newtonian fluid.
Incredible Magic Liquid Goo [albotas]