of a small candle with an empty glass. To the contrary, if air is blown over a wood fire (as with bellows), the fire is activated by the introduction of more air. Some chemicals, such as fluorine gas, perchlorate salts such as ammonium perchlorate, or chlorine trifluoride, act as oxidizers, sometimes more powerful ones than oxygen itself. A fire based on a reaction with these oxidizers can be very difficult to put out until the oxidizer is exhausted; that leg of the fire triangle cannot be broken by normal means (i.e., depriving it of air will not smother it). In certain cases such as some explosives, the oxidizer and combustible are the same (e.g., nitroglycerin, an unstable molecule that has oxidizing parts in the same molecule as the oxidizeable parts). Reaction is initiated by an activating energy — in most cases, it is heat. Several examples include friction, as in the case of matches, heating an electrical wire, a flame (propagation of fire), or a spark (from a lighter or from any starting electrical device). There are also many other ways to bring sufficient activation energy including electricity, radiation, and pressure, all of which will lead to a temperature rise. In most cases, heat production enables self-sustainability of the reaction, and enables a chain reaction to grow. The temperature at which a liquid produces sufficient vapor to get a flammable mix with self-sustainable combustion is called its flash-point. Extinguishment of the fire To stop a combustion reaction, one of the three elements of the fire triangle must be removed. Without sufficient heat, a fire cannot begin, and it cannot continue. Heat can be removed by the application of a substance which reduces the amount of heat available to the fire reaction. This is often water, which absorbs heat for phase change from water to steam. Introducing sufficient quantities and types of powder or gas in the flame reduces the amount of heat available for the fire reaction in the same manner. Scraping embers from a burning structure also removes the heat source. Turning off the electricity in an electrical fire removes the ignition source. Without fuel, a fire will stop. Fuel can be removed naturally, as where the fire has consumed all the burnable fuel, or manually, by mechanically or chemically removing the fuel from the fire. Fuel separation is an important factor in wildland fire suppression, and is the