A
fuel is any material that can react to release
energy from a potential form into a usable form. In most cases this is just something that will
burn. For example,
coal was burnt by
steam trains to heat water to provide power, and
natural gas is burnt to provide
heat to run power plants. However a fuel is not necessarily combustible. For example, in a
nuclear reaction a fuel will undergo
fission. This still provides a useful source of
energy but not via combustion.
In the bodies of most animals, the fuel sources are carbohydrate, fat, protein, which supplies the energy for muscles.