The term fuel oil generally includes any liquid fuel that is burned in a furnace or boiler to generate heat, or used in an engine to generate power. However, it doesn't usually include other liquid oils, such as those with a flash point of approximately 42 °C (108 °F), or oils burned in cotton- or wool-wick burners. In a stricter sense, the term fuel oil refers only to the heaviest commercial fuels that crude oil can yield, that is, those fuels heavier than gasoline (petrol) and naphtha.