A pyrophoric substance (from Greek purophoros, "fire-bearing") will ignite spontaneously; that is, its autoignition temperature is 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 a few exceptions) nitrogen. Most pyrophoric fires should be extinguished w… (More on Pyrophoricity)