Why do fire fighters use foam?

There are several methods firefighters use to put out fires. Fire needs 3 things to survive, called "the triangle of fire", heat, oxygen, and fuel. All firefighting techniques remove one of those elements to put out the fire. The first is to spray water or fire retardant on the fire, either using a fire hose or a special aircraft. They smother the fire, removing oxygen, and water cools it. Another method is a fire break. A fire break is a space with no fuel. Therefore, the fire cannot burn past the fire break, and the fire eventually exhausts its fuel and burns out. Those are the two most common ways to put out a fire.

Recent thinking on fires treat them as a four legged stool, now including "chain reaction." Some newer firefighting methods simply break the chain reaction to stop the fire. Chemicals are often used on fires, but the best know way is to use a fine spray of water. Although the water cools the fire (and resulting steam remove oxygen), a spray or even foam can separate the fuel from the air, starving the fire as it rapidly cools.