Tie a copper wire around a penny tightly, making sure the stripped end is touching the copper. You should wrap the wire around the penny a few times.
Place the penny and copper wire unit in the slit of your lemon.
Pierce the lemon with a galvanized nail, in the opposite side of the lemon as the penny.
Do the same thing with another lemon, penny, copper wire, and galvanized nail.
Place the two lemon side by side.
Wrap the copper wire from one lemon around the galvanized nail of the other lemon.
Connect the two wires to a light bulb, LED, small clock, or any small battery powered device
Science Behind It
The lemon juice is very acidic. Its pH level ranges from 2-3, anything less than 7 on the pH scale is considered acidic. The common battery pH level is 1. The reason why the lemon juice works as a battery is because the acid in the lemon juice reacts with the copper to create an electrochemical reaction which generates a small potential difference which exchanges electrons to reach a lower energy state, this creates the electric current to power things.
Tips
Leaders should complete a battery at home to see how much you can power with one lemon, that way leaders do not try to use devices that use too many volts. Another thing that members can try is attaching more lemons together so the battery's voltage increases. To do this you would connect the lemon by wrapping copper wire around a galvanized nail and and connecting it to another galvanized nail in another lemon. A variation to this project is members can also do this same procedure with an other fruit or vegetable with a high pH level.
Make sure when you wrap the wire, it's as tight as possible, because if it is not tight the circuit would not be completed. Also, make sure the nail and the penny don't touch because it will cause the battery to short circuit. If your lemon is not juicy enough, roll it between your hands to get more juices out of it. You also may need more than two lemons to conduct an electrical current.