Gases that are fuels burn easily because the gas and the oxygen can mix easily and the heat generated encourages further burning. For a liquid to burn, the heat from the fire must boil off some of the fuel so that it becomes a vapour and can mix with the oxygen in the air. For a solid fuel to burn, the heat from the fire has to decompose fuel into vapours that can mix with oxygen in the air. Oxygen Oxygen is a colourless gas that occurs in the air around us. Oxygen is essential to human life and to most plants and animals as well.
Fire occurs when a fuel combines with the oxygen in the air and gives off heat.
So there is generally enough for a fire to burn. Heat is essential to the burning process. If fresh fuel is not heated, then a fire cannot spread. The effects of fire result from the movement of heat away from the flame and into an adjacent area, where there is fresh fuel. Take the heat away and the fire goes out.