Fossil fuels are burned the thermal energy of burning fuel turns water into steam

When fossil fuels such as coal are burned, the thermal energy released can be used to heat water which on boiling produces steam. Steam under pressure can be used to do useful work. This is an example of the first law of thermodynamics which states that energy cannot be created and neither can it be destroyed, but it can be changed. Change in energy can be used by people to do useful work. The steam engine used steam under pressure and brought about an industrial revolution in Britain in the 19th Century.