What is fire following?

Answer 1

Oxygen? or fuel?

I am not sure I understand your question.

Engineers and fire fighters have used for many years what is called the fire triangle to describe fires and how to prevent them. The fire triangle is:

oxygen

fuel

source of ignition (heat)

If you can eliminate any one of the three, you have no fire. Fire fighting agents are designed to smother the fire (get rid of oxygen) or cool it (water does this well). But this doesn't always work with some fires, particularly chemical fires that may supply their own oxygen, fuel and heat.

So in the last 20 years or so fire fighting agents have been developed that actually stop the chemical reaction that is occurring in a chemical fire.

Also electrical (source of ignition) fire usually will re-flash (start again) unless you cut off the electricity. So fire fighters need to know if it's an electrical fire.