Fuels come in many different forms. Some are gases, like methane and propane, the fuels we burn in a gas cooker or a camping stove. Some are liquids like petrol and diesel, the fuels we burn in engines. And some are solids like wood and coal, the fuels we burn in fireplaces. But many other materials can be fuels.
Many plastics are fuels.
Paper and cotton are fuels. 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.