ABSTRACT
Granular materials can flow like liquids under some conditions, yet remain rigid under others.1 Their intrinsic complexity is a consequence of the fact that they are composed of many pieces that can assemble in large structures that move (or refuse to move) collectively in surprising ways. Understanding this requires new paradigms and tools that go beyond the traditional domains of solid state physics, engineering and material science. It is forcing us to rethink established classifications of matter.
