A pyrophoric substance (from Greek purophoros, "fire-bearing") will ignite spontaneously; that's, its autoignition temperature's below room temperature. Examples are iron sulfide and many reactive metals including uranium, when powdered or sliced thinly. Pyrophoric materials are often water reactive as well and will ignite when they contact water or humid air. They can be handled safely in atmospheres of argon or (with few exceptions) nitrogen. Most pyrophoric fires should be extinguished with… (More on Pyrophoricity)