Instead of burning fuel like conventional engines, hydrogen fuel cells work through an electrochemical process. To produce electricity, hydrogen atoms are ionized on one side of an electrolyte membrane. While protons slip through, electrons must take the long way around through an external circuit, creating an electrical current as they move. Once the electrons reach the other side and pair off with the protons, the hydrogen combines with oxygen in the air, resulting in a little bit of heat and water as byproducts.
Fuel economy labels have been around for about three decades. But now it's time for new labels, and the EPA calls it "the most dramatic overhaul in the history of EPA's labeling program."
How does a gas pump know when you've got a full tank? Read this simple explanation of the mechanics that cause a gas pump to shut off when the tank is full.