Oh geez, thats silly.
I too did the fruit battery back in gradeschool, for a science project.
As I recall, it took like 15 or 20 lemons to light a penlight bulb. Used copper and zinc strips as the anodes and cathodes.
Heh.. It was neither small, nor cheap.. Lemons are expensive if you don't have a tree in your backyard --
When cryptography is outlawed, bayl bhgynjf jvyy unir cevinpl.