After transforming the primary energy resources, the secondary energy in the form of liquid and gas fuels of petroleum products, electricity, heat, as well as biofuels can be produced. For example, chemical energy found in fossil fuels such as coal or natural gas is used in steam power plants to produce electricity. Electricity is also produced by other means such as the kinetic energy of flowing water, wind, solar photovoltaic, and geothermal power. Hydroelectric power is produced when falling water turns an electromagnet that generates electricity. Nuclear power is produced when neutrons bombard heavy atoms, which release energy in a process called fission. The world's power demand is expected to rise considerably. Cogeneration is the production of more than one useful form of energy (such as process heat and electric power) from the same energy source.