While learning about Direct Current, our 1st year apprentices created batteries out of lemons. They were able to demonstrate how the citric acid acts as the electrolyte and facilitates the chemical reaction that occurs in batteries. A galvanized roofing nail and a piece of copper act as the electrodes. They were also able to measure the differences in voltage as they connect the lemon batteries in series and parallel. Interestingly, everyday lemons create enough energy to light up an LED.
The energy comes from the oxidization of the zinc as it dissolves in the citric acid. This creates a exchange of electrons with the acid. They were able to create a little under 1 volt per lemon. By wiring several lemons in series, enough energy was harnessed to light up an LED that was visible even with the lights on in the room. This is a great science experiment to try at home. The video below explains exactly how the lemon battery works.