Fire needs three things at once — fuel, oxygen and heat — the classic "fire triangle". Modern fire science adds a fourth leg: the chain reaction, the self-sustaining cascade of free-radical combustion reactions that keeps the flame propagating once ignition happens. Remove any one of the four and the flame collapses — that is exactly what each extinguishing agent below does.
- Water — cools by absorbing heat, mostly as its huge latent heat of vaporization (~2260 kJ/kg turns to steam). Excellent on ordinary combustibles (wood, paper, cloth). Never on a hot oil/grease fire — water sinks under the oil, flash-boils instantly and blasts burning oil into the air (explosive splash). Never on live electrical equipment — water conducts current and risks electrocuting whoever holds the hose.
- CO₂ — a heavier-than-air inert gas that blankets the fire and displaces oxygen, dropping the local O₂ concentration below the ~15% most flames need to sustain combustion. Leaves no residue, so it's safe on live electrical fires and delicate equipment.
- Foam — an aqueous film that floats on top of a fuel surface, physically sealing it off from atmospheric oxygen while its water content also cools. Effective on liquid-fuel fires, but it contains water and conducts electricity, so it is not used on live electrical fires.
- Dry chemical powder — fine mineral powder (e.g. sodium or potassium bicarbonate) that decomposes in the heat, releasing radicals that interrupt the flame's chain reaction directly. Non-conductive and effective on nearly every class of fire, including electrical.
Choosing the wrong agent for the fire type below flares the flame up temporarily instead of extinguishing it — exactly as it would in reality.