Why don't sand dunes collapse? How does sand flow in an hourglass? How is it possible to empty a silo of all its wheat? What's a ceramic? The answer to all these questions can be found in the science of the complex organizations of matter, a science which is pluridisciplinary. Étienne Guyon, head of the École Normale Supérieure, and Jean-Paul Troadec, a researcher, present the characteristics of grain matter, the rules by which it is organized (in both crystal and fluid) as well as its movements (in silos as in avalanches).