A simple Battery

Materials:
A Copper coin
An Iron nail
Copper wire
Fresh Lemon
A small LED light bulb (low voltage)
Sand paper
Knife

Procedure
Apply sand paper on the copper coin and the iron nail (to clean out the oxidants)
Cut 2 small openings on different sides of the lemon
Insert a coin into one side of lemon (indicated left hand side)
Insert a nail into another side of lemon
NOTE: let the coin and nail contact with the lemon's carpel
Connect the copper wire to two sides of LED light bulb
Another sides of copper wire connect to the coin and the nail

Result
The electricity is to weak to generate a LED bulb, so it is really hard to see the light. However, it is observable in the dark