“A dilatant material is one in which viscosity increases with the rate of shear.” (wikipedia)

In other words, a dilatant material will become “thicker” or “harder” when it is hit. This is counter-intuitive… imagine a liquid that becomes solid when it is hit, then quickly returns to a liquid.

A simple example of a dilatant material is called “oobleck“. Oobleck is a mixture of cornstarch and water. Click here to learn how to make oobleck at home.

There is currently research going on that uses dilatant materials for flexible body armor. When a bullet strikes the fluid, it immediately becomes hard, protecting the person wearing it. The armor is full of polyethylene glycol with nano-particles of silica suspended in it. For you super-nerds: polyethylene glycol is a relative if ethylene glycol (anit-freeze), and is used to thicken shampoo and cosmetics.