Why can you walk on a mixture of cornstarch and water?

The answer

A cornstarch-water mixture (oobleck) is a non-Newtonian fluid whose viscosity changes with applied force. Under slow, gentle pressure, the suspended starch particles can rearrange and the mixture flows like a liquid. Under sudden impact, the particles jam together into a rigid structure that behaves like a solid. This 'shear thickening' effect occurs because the force drives water out from between particles faster than it can flow back in, temporarily locking them in place. The effect reverses the instant the force stops.