How is potato a good conductor of electricity?
A potato will work as a conductor in an electrical circuit if the voltage is high enough. You, however, want one to work as a battery. The potato has electrolytes in it that will encourage electron flow if a copper strip (or a penny) and a zinc strip (or a galvanized nail) are inserted into the potato to complete a circuit. A single potato will probably not provide enough current to light a light bulb, but may light a low current LED.