Foam is an effective smothering agent, primarily used for liquid fires. It works by flowing over the surface of the liquid fuel oil and isolating the fire from the air; it also prevents re-ignition due to the foam stability. Typically, the foams are I Protein-Based ii Foam fire extinguishers, which are suitable for use on class A solid combustible fires and class B flammable liquid fires, creating a cooling blanket effect, smothering burning materials and preventing reignition. Unless specifically labeled as class F extinguishers, they should not be used on-chip pan fires.