Two sides are on the fire tetrahedron: one is fuel and the other is heat. It looks like a triangle with each side representing a different thing that you need to start a fire. Oxygen and a long-term chemical reaction are the two other things Carroll says are on the other two sides. He says that, with the exception of granite and asbestos, most materials can be made more or less flammable by deleting one or more sides of the fire tetrahedron. This makes them more or less flammable.