How to make a battery from a lemon

Take 4 zinc-coated nails, 4 copper coins and 4 lemons.
Insert a nail into one side and a coin into other side of each lemon.
Connect lemons together with a wire. A nail in one lemon should be connected to a coin in another.
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You can turn on a LED with this battery but you must determine “+” and “-“ connections on it. If you look closely at the LED you will notice that one of the wires comes out of “flat” spot. That wire must be connected to the “-“ side of the battery.