The acid in the leather, fabric, or cardboard discs will try to dissolve some of the metal in the zinc discs. To be dissolved, a zinc atom must give away 2 of its electrons. The zinc atom then becomes an ion: Ions can "go where they want to" in the liquid acid. They go through the leather/cardboard disc, and meet the copper disc. The zinc ions are "missing" the two electrons they left behind, so when they get to the copper disc, they each "steal" two electrons to replace the missing ones. When they "steal" electrons, the zinc ions turn back from ions to "normal" zinc atoms. Saltwater can also be used in the cardboard, fabric, or leather, in a voltaic pile using copper and zinc plates.