Examples of granular matter: multicolored plastic balls in a playground, gravel, sesame seeds and lentils (clockwise from top left). Images by Julia Kertesz, Martin Olsson, Jitender Kumar and Zyance ...

A stroll on the beach can mean sinking your toes into smooth sand or walking firm-footed on a surface that appears almost solid. While both properties are commonplace, exactly what it is that makes ...