Dry-cell batteries generate electric power from magnetized particles through a slow-acting chemical reaction. Normally the user does not play any role in this process. But Nakabayashi Co., Ltd. has just put on sale “Mizudenchi NOPOPO,” reusable batteries that can be activated by inserting water (plastic dropper provided). Their main advantage is that unlike ordinary dry-cell batteries that gradually discharge over time until they become flat, this type can be stored without use approximately 20 years, making them ideal for back-up use in emergencies. The manufacturer claims that if water is not available, most liquids, such as fruit juice or beer — or even saliva — can be used in a pinch. They are much lighter than ordinary batteries and contain no substances harmful to the environment. A set of three AA size batteries sells for 630 yen ($7.8).