They work by smothering the fire: when you spread a thin layer of foam over a fire, you cut the fuel off from the oxygen around it. Foam extinguishers also help to absorb heat, since the cool foam they release contains a lot of water.

What makes a fire extinguisher work?

Most fire extinguishers work by separating the fuel from the oxygen. The oxygen comes from the air. It is the same oxygen we breathe. Since the oxygen has to be in contact with the fuel, if you can coat the fuel with something that keeps the oxygen away, the fire will go out.

How do fire extinguishers work step by step?