A fuel cell makes electricity using the energy released by mixing fuel with air, a reaction which creates water and sometimes also carbon dioxide. The most common fuel for fuel cells is hydrogen, which when reacted with oxygen from air produces only water. Fuel cells work like a battery being constantly fed with fuel so it never runs out (as long as you have enough fuel). Fuel cells are an important part of the hydrogen economy. Hydrogen molecules are found in substances such as methane, water and biomass but in all cases some energy is required to extract it. There are two common ways to produce hydrogen - it can be separated from most fuels like oil, gas, coal in a process called steam reforming, or it can be extracted from water using a process called electrolysis. If the hydrogen is separated from fossil fuels, carbon dioxide is released. If the energy used to extract it from water via electrolysis comes from solar or wind, then the hydrogen produced is benign because no emissions are released. Hydrogen can also be separated from renewable biogas, which means the carbon emitted is not of fossil origin and therefore is part of the natural carbon cycle. 