Which of the following is not an element of the fire tetrahedron?

The fire tetrahedron is a conceptual model that helps understand the components necessary for combustion. It consists of four elements: heat, fuel, oxygen, and a chemical chain reaction. Each of these elements plays a crucial role in sustaining a fire; when any one of these components is removed, combustion will stop.

In this context, water does not form part of the fire tetrahedron. While water is commonly used as an extinguishing agent, it does not represent a necessary component for combustion. Instead, water serves to cool the heat, displace oxygen, or interrupt the chemical reaction. Understanding that water is not a part of the tetrahedron is essential for grasping basic firefighting principles and the nature of fire behavior.