The potato is the electrolyte, not the battery
A potato can help run a small digital clock because its moist interior lets charged particles move between two metal electrodes. That makes it an electrolyte: a conducting medium that helps complete an electrochemical reaction. The potato itself is not generating electricity from its starch.
The electrodes must be different metals. In a common setup, a galvanized nail supplies zinc—the thin coating that protects the steel nail—while a second electrode is made of a different metal, such as copper. Zinc atoms release electrons as they react; those electrons travel through the external wire toward the other electrode. The movement of electrons is the electric current that can power the clock.
Why a clock can run on such a small cell
The potato mainly provides a damp, mildly acidic environment for the chemical reactions and for ions to move inside the cell. The nail and the second metal provide the materials that drive the voltage. Connecting them to the clock closes the circuit, allowing current to flow through its electronics.
One potato cell usually supplies only a modest voltage, so a particular clock may need several cells connected in series—end to end, so their voltages add. The clock’s low power demand makes this more plausible than running a motor or a bright lamp. As the zinc is consumed and the reactions slow, the output falls; the potato is a temporary cell, not a renewable source of energy. The striking part is that the useful electricity comes from a chemical reaction between metals, while the ordinary potato mostly helps that reaction happen.
