EARTH - Shifting sands: Force is the key to granular state-shifting

Ever wonder why sand can both run through an hourglass like a liquid and be solid enough to support buildings? It's because granular materials -- like sand or dirt -- can change their behavior, or state. Researchers have found that the forces individual grains exert on one another are what most affect that transition.
http://www.sciencedaily.com/releases/2013/02/130217085037.ht