Which of the following is not an essential ingredient for all ordinary fires?

The fundamental components for a fire to occur are commonly known as the fire triangle, which consists of heat, fuel, and oxygen. Each of these elements plays a critical role in the ignition and sustaining of a fire: heat raises the material to its ignition temperature, fuel provides the substance that burns, and oxygen supports the combustion process.

While an ignition source may certainly initiate a fire, it is not a requirement for every ordinary fire situation. For example, if fuel is preheated to its ignition point, it could ignite spontaneously without a direct ignition source. Therefore, while having a means to ignite the fire is helpful, it is not strictly necessary to classify a scenario as capable of sustaining a fire.

This understanding clarifies why the ignition source is not considered an essential ingredient in the fire triangle itself, as it is the combination of heat, fuel, and oxygen that creates the necessary conditions for an ordinary fire to exist.