No worries about batteries running down in this calculator or that the light wont be strong enough to power it up! This unique desk tool runs on ordinary tap water. Its a cost-saving, convenient alternative to conventional batteries simply fill the replaceable battery with a few drops of water for 2-3 months of use. When the display starts to fade, just add more water! The water-powered battery lasts a lot longer than alkaline batteries and wont pose a threat to the environment. With all the features of a standard calculator, this innovative version is a practical accessory for home, the office or school.