A fire is not a substance but a process, and it persists only while four things are present at once: fuel in a gaseous or finely divided form, an oxidiser (normally the roughly 21 percent oxygen in air), enough heat to keep converting fuel into vapour, and the self-sustaining chain of free-radical reactions inside the flame. The older fire triangle names the first three; the tetrahedron adds the chain reaction, which is the leg a chemical agent such as a clean-agent gas interrupts. The usable rule is that any one missing leg ends the fire, so every change to a fire can be classified by the leg it attacks: cooling until vapour production stops, smothering to cut oxygen, removing or separating fuel, or breaking the flame chemistry. The common confusion is treating fire as an object to be pushed away rather than a reaction to be starved; a fire that goes out always goes out because one leg went missing. After this Concept you can name which leg of the tetrahedron a given change removes, and predict whether the fire keeps burning.
Fire safety science models combustion as a set of interdependent necessary conditions: the fire triangle defines fire as requiring fuel, oxygen, and heat, such that removing any one element causes th…