Practical it is not, but this gadget sure is nifty! The Hydro Power Calculator ($9.95) is powered by water - all you have to do is fill the "batteries" with tap water and you're ready to calculate.
Water-powered battery consists of two electrodes made of special alloys, which, when immersed in water, produce electricity through a chemical reaction.
Simply refill the battery as the water evaporates - typically every 2 to 3 months. The battery has a minimum continuous life of 2 years.