Ductility is the capacity of a material to deform permanently in response to stress. Brittle materials, such as glass, cannot accommodate concentrations of stress because they lack ductility, and therefore fracture rather easily. Metals that are ductile at ordinary temperatures include aluminum, cadmium, indium, and tin; the transition metals titanium, vanadium, iron, nickel, copper, zirconium, niobium, molybdenum, palladium, silver, tantalum, rhenium, platinum, and gold; the alkali metal cesium; the alkaline earth metals magnesium and calcium; the rare earth metals yttrium, lanthanum, cerium, praseodymium, europium, gadolinium, terbium, thulium, and ytterbium; and the actinons thorium and uranium. (Zinc, tungsten, and iridium are normally brittle, but can be made ductile at high temperatures).