Not all fires are the same, and they are classified according to the type of fuel that is burning. If you use the wrong type of fire extinguisher on the wrong class of fire, you can, in fact, make matters worse. It is therefore very important to understand the four different fire classifications.
Class A - Wood, paper, cloth, trash, and plastics - Solid combustible materials that are not metals. (Class A fires generally leave an Ash.)
Class B - Flammable liquids: gasoline, oil, grease, and acetone - Any non-metal in a liquid state, on fire. This classification also includes flammable gases. (Class B fires generally involve materials that Boil or Bubble.)
Class C - Electrical: energized electrical equipment - As long as it's "plugged in," it would be considered a class C fire. (Class C fires generally deal with electrical Current.)
Class D - Metals: potassium, sodium, aluminum, and magnesium - Unless you work in a laboratory or in an industry that uses these materials, it is unlikely you'll have to deal with a Class D fire. It takes special extinguishing agents (Metal-X, foam) to fight such a fire.
Most fire extinguishers will have a pictograph label telling you which classifications of fire the extinguisher is designed to fight. For example, a simple water extinguisher might have a label like the one below, indicating that it should only be used on Class A fires.
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