I remember this from the old 'potato clock' science kits from the 80s. They had a copper cathode and zinc anode that when inserted into a potato (most fruit/veg

I remember this from the old 'potato clock' science kits from the 80s. They had a copper cathode and zinc anode that when inserted into a potato (most fruit/vegetables or acidic liquids also worked) generated enough power to run a small LCD clock. In a potato, the copper cathode always left a green spot.