Ever since the Iron Age, we have known that beating certain metals repeatedly makes them harder, but have not until now understood exactly why. US researchers have discovered finally that three is a magic number in this process in which defects known as dislocations get knotted toughening up the battered metal. Vasily Bulatov and colleagues at Lawrence Livermore National Laboratory in California, realized that the standard explanation for metal hardening was ignoring a key element. Find out more…