concept

Fire tetrahedron

Fire needs four things at once, not three. The familiar fire triangle lists fuel, oxidiser and heat; the tetrahedron adds a fourth face, the self-sustaining free-radical chain reaction whose stream of H•, OH• and O• fragments is what actually carries combustion forward. Halon-type clean agents do not smother a flame or cool it below its ignition temperature; they scavenge those radicals, attacking that fourth leg directly. Remove any one of the four and the flame dies, which means there are four ways to put out a fire rather than three.