Sands of Mars: "Granular physics is the science of grains, everything from kernels of corn to grains of sand to grounds of coffee. These are common everyday substances, but they can be vexingly difficult to predict. One moment they behave like solids, the next like liquids.
"Consider a dump truck full of gravel. When the truck begins to tilt, the gravel remains in a solid pile, until at a certain angle it suddenly becomes a thundering river of rock."
"The problem is, even here on Earth 'industrial plants don't work very well because we don't understand equations for granular materials as well as we understand the equations for liquids and gases,' says James T. Jenkins, professor of theoretical and applied mechanics at Cornell University in Ithaca, N.Y."