A MAGICIAN slips a knife down into a sand-filled bottle, gives the bottle a little shake and then, hey presto, uses the knife handle to pick the bottle up and swing it around his head.
Intrigued by this old magic trick, Harry Swinney and colleagues at the University of Texas at Austin put thousands of tiny glass beads into a tube of water, and gradually packed them in more tightly. The team found that when the beads occupy 59 per cent of the available volume they lock together, suddenly making the mixture harder to penetrate – a transition akin to…
