OK, water isn't technically a biofuel. It's a vital natural resource without which life wouldn't exist. But thanks to a deceptively simple technology, water could one day be a conceivable source of fuel.
The simple process of electrolysis, in which electric current is passed through water, breaks the liquid into its base elements: hydrogen and oxygen [source: Nave]. Hydrogen is an excellent fuel -- it carries three times the energy per pound of gasoline and combusts without the harmful emissions of petroleum fuels [source: Stanford].
But hydrogen production and storage are problematic. Moving large amounts of super-light, highly combustible gas around the world could pose major safety issues, and the amount of hydrogen needed to power a car for a long trip would require an impractically heavy fuel tank to keep enough fuel onboard in a safe manner [source: Planet].
Hydrogen's far from a lost cause, though. One technology, made famous by the mysterious Garrett Water Carburetor, involves mounting a hydrogen-producing cell on the vehicle and running it with electricity from the engine's generator. Modern versions of this idea inject hydrogen into gasoline-powered engines, generating cleaner emissions and better mileage. The technology has some cost, reliability and development hurdles to overcome, but it's possible that part of your car's near-future fuel will come from your home's spigot [source: Brooks].