In 1897 the Swiss physicist Charles Edouard Guillaume discovered an iron-nickel alloy with the unusual property that it did not expand when heated. This alloy, known as Invar, is now widely used in applications as diverse as watch springs and the "shadow masks" found in televisions and computer monitors. These masks prevent the electron beam in these devices from hitting the wrong phosphor dots on the screen; if they expanded when heated they would distort the picture. Although Guillaume won the 1920 Nobel prize for his discovery, a proper microscopic explanation of the unusual properties of Invar has had to wait until recent computer simulations by Mark van Schilfgaarde of Sandia National Laboratories in the US and Igor Abrikosov and Borje Johansson of Uppsala University in Sweden (Nature 400 46).