For a fire to take shape (as well as continue), the following three conditions should be met:
1. A combustible material
2. A specific temperature at which the above material would burn
3. Some fuel (mostly oxygen) to aid the burning
When petrol/gasoline is burning, the petrol/gasoline is the combustible material, and, the atmosphere provides the fuel (oxygen). The presence of the above 3 elements together is called the “fire triangle”. Each of these 3 elements have to be present to start a fire, and for the fire to continue.
Sometime, one element may aid in producing the other element.
Lets consider an example of how one element aids in the production of another element. Lets say, we want to burn a piece of cloth. The cloth is lying there in front of us. There is enough oxygen in the atmosphere. But, the cloth does not burn. So, we now douse the cloth in gasoline. Still, there is no fire. Now, we burn a matchstick – by rubbing a matchstick against the matchbox/matchbook. This rubbing causes a minor increase in temperature. At this temperature, the matchstick lights up. The lighted matchstick creates still higher temperature. Now, when the matchstick is touched to the doused (with petrol/gasoline) piece of cloth, the petrol/gasoline also starts burning. This further increases the temperature, which causes the cloth to start burning, which can now produce still higher temperature. So, here, friction caused an increase in temperature. This increase in temperature converted the matchstick into a combustible element. This in turn raised the temperature further, and so on.
Now, that we know that for a fire to be sustained, we need all 3 arms of the fire-triangle. This forms the fundamental principle behind all fire-fighting techniques.
If we have to control a fire, the way to extinguish it is to remove atleast one arm of the fire-triangle. Sometimes, we might want to simultaneously attack 2 arms also. That is because, we know that one arm could aid the other arm. So, by trying to simultaneously fight two arms, we might achieve the results faster.
It’s a matter of experience and the situation which decides which arm to fight. Usually, you try to remove that arm, which is the easiest to remove.