Sand. A single grain is tiny, but solid, and shares the physical properties of other solid matter. But pack or transport millions of grains together - as modern society does with coffee grounds, flour and industrial chemicals - and granular materials act differently, baffling engineers. They take the shape of their containers and flow freely, like liquids. In certain circumstances, they exert pressure like a gas. The basic lack of behavioral knowledge contributes to wasted resources and energy, as well as erosion and other natural phenomena.