A potato battery is a type of electrochemical cell
. An electrochemical cell converts chemical energy into electrical energy. In the potato battery, there is a transfer of electrons between the zinc coating the galvanized nail that will be inserted into the potato and the copper wire that will be inserted another part of the potato. The potato conducts electricity, yet keeps the zinc ions and copper ions separate, so that the electrons in the copper wire are forced to move (generate current). It's not enough power to shock you, but the potato can run a small digital clock.