Many natural (e.g. sand, astroid belts) and industrial (e.g. grains, gravel) systems consist of essemblies of particles which are both very rigid and too heavy to be sensitive to thermal fluctuations. These systems have peculiar mechanical properties - depending on external conditions, they sometimes behave as solids, and at others - as fluids. For instance, we can build a sand castle or make a pile of sand, which are solid (albeit rather unstable) structures that carry their own weight, but sand can also flow as in an hour-glass. In my research I employ both theory and numerical simulations to understand the rich physics of these driven systems.