it expands over 4,000 times). This leaves the fire without enough of the combustive agent, and it dies out. The vaporization of water also absorbs heat; it thereby cools the smoke, air, walls, and objects that could act as further fuel, and thus prevents one of the means by which fires grow, which is by "jumping" to nearby heat/fuel sources to start new fires, which then combine. Water extinguishment is thus a combination of "asphyxia" (cutting off the oxygen supply) and cooling. The flame itself is suppressed by asphyxia, but the cooling is the most important element in mastering a fire in a closed area. Water may be accessed from a pressurized fire hydrant, pumped from water sources such as lakes or rivers, delivered by tanker truck, or dropped from water bombers, which are aircraft adapted as tankers for fighting forest fires. An armored vehicle (firefighting tank) may be used where access to the area is difficult. Open air fire For outdoor fires, the seat of the fire is sprayed with a straight spray: the cooling effect immediately follows the "asphyxia" caused by vaporization and reduces the further amount of water required. This is because water droplets, upon forming in to water mist, increase their surface area by a large magnutude, greatly increasing the endothermic cooling effect and robbing the fire of oxygen. A straight spray is used so the water arrives massively to the seat of the fire before it vaporizes. A strong spray may also have a mechanical effect; it can disperse the combustible product and thus prevent the fire from starting again. Spray is always aimed at a surface or an object. For this reason, the strategy is sometimes called a two-dimensional or 2D attack. An outdoor fire is always fed with air, and the risk to people is limited as they can move away from it, except in the case of wildfires or bushfires where they risk being easily surrounded by the flames. It might, however, be necessary to protect specific objects like houses or gas tanks against infrared radiation, and thus to use a diffused spray between the fire and the object. Breathing apparatus is often required as there is still the risk of inhaling smoke or poisonous gases. Closed volume fire Until the 1970s, fires were usually attacked while they declined, using the same strategy as for open air fires. Now fires are attacked