In bird colonies, schools of fish and cycling pelotons, significant interactions occur between individuals through the surrounding fluid. These interactions are well understood in fluids such as air and water, but what happens when objects move through something like sand? It turns out that similar interactions occur in granular materials—things like soil or sand—and they play a crucial role in everyday contexts. Think of plows cutting through farmland, animals burrowing underground, roots pushing through soil, or even robots exploring the surface of Mars.
