Liquid fuels are combustible or energy-generating molecules that can be harnessed to create
mechanical energy, usually producing
kinetic energy; they also must take the shape of their container. It is the fumes of liquid fuels that are flammable instead of the fluid. Most liquid fuels in widespread use are derived from
fossil fuels; however, there are several types, such as hydrogen fuel (for
automotive uses), ethanol, and biodiesel, which are also categorized as a liquid fuel. Many liquid fuels play a primary role in transportation and the economy.