Fuel is generally defined as any material that contains stored energy and can be converted into usable energy, typically through a controlled chemical reaction, such as combustion. The primary function of a fuel source is to generate heat, which can then be used to create mechanical work or electrical power. Fuels are classified based on their chemical composition and the mechanism of energy release; examples include chemical fuels (like hydrocarbons, coal, and natural gas), which undergo exothermic combustion, and nuclear fuels (such as uranium compounds), which release energy through controlled nuclear decay. The efficiency and environmental impact of a fuel source are determined by its energy density, the byproducts of its combustion, and the ease with which it can be stored and transported.