Fires are generally extinguished by removing part of the processthat makes them keep going, sometimes called the "firetetrahedron": fuel, oxygen, heat, chemical reaction. For
… example, putting water on a big fire causes the water to turnto steam, which is a cooling process, removing the heat from thefire. Water that gets onto the surface of the fuel also reduces theamount of oxygen available for burning and changes the fuel byraising its ignition temperature. It's harder to light wet fuelthan dry fuel. Some fire extinguishing methods involve "smothering" the fire, i.e.cutting off the supply of oxygen. Carbon dioxide and foamextinguishers are examples of extinguishers that work on thisprinciple. They also absorb much of the heat from the fire, whichprovides a secondary, lesser fire extinguishing benefit. Some fire require adding substances to the fire that interrupt thechemical reaction. Most of these are halogenated organics - broadlyreferred to as Halons. At high temperatures, halons decompose torelease halogen atoms that combine readily with active hydrogenatoms, quenching flame propagation reactions even when adequatefuel, oxygen, and heat remain. The preferred halogens are bromineand iodine although chlorine and fluorine can also help quenchfires. ( Full Answer )