Which element is NOT part of the fire tetrahedron?

The fire tetrahedron is a model used to explain the different components necessary for fire to ignite and sustain itself. It consists of four essential elements: fuel, oxygen, heat, and a chemical chain reaction.

In this context, water is not one of the elements in the fire tetrahedron. While water is commonly used as an extinguishing agent to put out fires, it does not contribute to the conditions required for fire; instead, it acts to remove one or more of the elements in the tetrahedron, effectively breaking the fire triangle and thus extinguishing the flame.

Fuel, oxygen, and heat are all critical components that must be present for a fire to start and continue burning. Removing any one of these elements will extinguish the fire. Water plays a different role in the context of fire safety and suppression techniques rather than being a fundamental element that allows for combustion.