Here is a fun little video on how to make batteries from pennies, cardboard and vinegar, or pennies, washers, cardboard and vinegar. Oh, and of course, electrical tape. Seriously this could be a handy thing if you badly needed a battery. It's probably a lot easier to keep some extra batteries around the house, but this was pretty impressive anyway.
This guy has a lot of interesting things on his site. This project reminded me of Phoebe's brother melting things.... :)