Using the fire tetrahedron (heat, fuel, oxygen, and the chemical chain reaction), fire can be extinguished by removing any one of these elements. Cooling removes heat, most commonly by applying water to lower the temperature below the fuel’s ignition point. Smothering reduces or displaces oxygen, such as using foam, carbon dioxide, or a fire blanket to prevent oxygen from reaching the fire. Starvation removes or isolates the fuel source by shutting off a gas valve, stopping a fuel spill, or separating combustibles from the fire area. Finally, chemical chain reaction interruption stops the combustion process at the molecular level; this is how dry chemical agents (like monoammonium phosphate) and some clean agents work by breaking the self-sustaining reaction. Removing any one of the tetrahedron components will stop the fire.