Put two large potatoes on a table. To ensure the potato batteries produce enough voltage to power the flashlight, wire two potatoes in a series.
Insert one 3-inch zinc nail into one potato and another 3-inch zinc nail into the second potato using your fingers. Position them so they are just off-center, with about half of the nail inside the potato.
Insert a 3-inch copper nail into the first potato and another 3-inch copper nail into the second potato. Insert the copper nails so they are off-center and about 2 inches away from the zinc nails.
Cut three 6-inch pieces of thin plastic coated-wire using a knife. Remove ½ inch of the plastic coating from the ends using wire cutters so the copper metal core shows.
Twist the end of the one of the wires around the top of the zinc nail in the first potato using your fingers. Twist the opposite end around of the wire around the top of the copper nail in the second potato.
Twist the end of the second strip of wire around the top of the copper nail in the first potato. Twist the end of the third strip of wire around the top of the zinc nail in the second battery.
Touch the ends of the two loose wires to the two terminals on the flashlight bulb. The bulb will illuminate. It may not be very bright, but you have made a series circuit using potatoes as a battery.