Why does it have to be 1 copper wire and one zinc nail for conducting electricity through a potato?

A potato acts as an electrolyte, enabling the flow of electricity between the two electrodes. The copper wire serves as the positive electrode (cathode), while the zinc nail acts as the negative electrode (anode). This setup allows for a chemical reaction to occur between the zinc and the electrolyte (the potato), generating a flow of electrons, which is what constitutes electricity. Using one of each metal ensures a potential difference that drives the current.