It’s not the potato making the clock run, it’s the metal. Potatoes are a favourite vegetable to use when conducting this electro-chemical experiment. You could also use a lemon. You cannot, however, substitute the metal used: copper and zinc. (You can use pennies and zinc-coated nails.)
   The electro-chemical reaction between these metals is what powers a veggie “battery”. The metal is stuck into the potato. Wires connect a small light bulb to the copper and zinc. Copper and zinc dissolve at different rates in potato “juice”. Zinc’s positively charged electrons are released into the potato. The negative electrons are left in the zinc metal, where they will flow to the copper if an external circuit is provided—it is in the form of wire and a light bulb (a very small light bulb). The different flow of positive and negative electrons—the positive back and forth from zinc to copper through the potato; and the negative electrons back and forth through the wire—is what powers your battery. When the negative electrons are moving back and forth, they pass through the light bulb. They heat up the wire filament inside and you get a wee bit of light—not enough to read by!