Oobleck

At some point, every physics student makes up a batch of Oobleck, a non-newtonian fluid. What's a non-newtonian fluid? Basically it's a compound that acts like water when under little or no sheer stress, but a solid when subjected to lots of stress. The most common demo shows someone stirring a beaker of the stuff with a spoon, then hitting the fluid with a hammer and watching it shatter. You can mix up a batch at home, it's basically cornstarch and water (WARNING: DO NOT DISPOSE OF OOBLECK IN THE GARBAGE DISPOSAL).

This demo however, is a vastly superior demonstration:



(via)