Most metals — from the steel used to build bridges and skyscrapers to the copper and gold used to form wires in microchips — are made of crystals: orderly arrays of molecules forming a perfectly repeating pattern. In many cases, the material is made of tiny crystals packed closely together, rather than one large crystal. The crystals tend to merge and grow larger if subjected to heat or stress. Now, researchers have found a way to avoid that problem. They’ve designed and made alloys that form extremely tiny grains — called nanocrystals — that are only a few billionths of a meter across.