Granular materials such as sand, powders, cereal grains, and gravel
are commonplace in the physical world around us. Being composed
of very large numbers of mesoscopic grains, they show
unique physical behavior that is unlike other materials such as fluids
and deformable bodies, which have been well studied in computer
graphics. In particular, granular materials disperse freely in free
fall, flow plastically under forcing, and yet settle in stable piles