Fire is an event, not a thing. Heating with wood or other fuel releases volatile vapors that can ignite quickly with oxygen in the air; resulting incandescent bloom of gas further heats the fuel, releasing more vapors and perpetuate the cycle.
Most of the fuel we use is obtained from solar energy trap. In the process of photosynthesis, the sunlight and the heat make chemical energy (in the form of wood or fossil fuels); fire use chemical energy to produce light and heat.
Oxygen supply affects the color of the flame. Low oxygen flame contains a lot of unburned fuel particles and emit yellow light. The high oxygen content of the fire is lit in blue.
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