Voltaic pile
The picture to the right shows a voltaic pile: it's a pile, or "stack" of discs, made from 3 different materials. Some of the discs are made from copper, some from zinc. Other metals can be used; but it's important that there are two different kinds of metal. There are also discs made from leather, fabric, or cardboard: these are the green discs in the picture. They must  be soaked, or "made wet", with something acidic.