The energy in fossil fuel is stored in the bonds between the atoms of hydrogen and carbon.
To break those bonds and release energy to do things like drive a turbine in a power plant,
we add enough heat to start a reaction in order to break some of the bonds in the fuel
(this is why we need a match to get a wood fire started). Then we introduce enough air to provide
enough oxygen to attract carbon and hydrogen atoms in the fuel—this keeps the reaction (fire!)
going. As the bonds in the fuel break, energy is released in the form of heat.
Much more heat is given off than we put in—that is the energy stored in the fuel. In the meantime,
the atoms that have been released as the energy is released combine with the free atoms
available in the air. These new combinations of atoms form new materials, mainly water
vapor and carbon dioxide.