This will teach indestructable build a simple electrolytic cell made with citrus fruits and objects found in any home.
Sorry for my English, I'm Brazilian so I do not speak English well.

Step 1: Hunt for Materials

citrus fruits (in my case I used the lemon Galician, but you can use oranges or even use citrus, opting for some acid like vinegar or sea water)

One or more penny

1 nail or screw (1 nails or screws (or whatever is made of a metal other than the money)

A multimeter or a voltmeter

a knife

Step 2: The Fruit

The principle of operation of the electrolytic cell is the transformation of chemical energy into electrical energy, so the more acidic the substance is, the more electricity will have our battery.
Make two cuts in the fruit, so we can fit the electrodes.

Step 3:

Attach the electrodes and your battery will be ready.
Using a lemon Galician and this structure got a voltage of 0.83 volts, but you can connect more batteries one other, thus increasing voltages.
Important: If you connect two or more batteries, make sure that the contact between them is perfect, but you will get with many very small voltage batteries together.

Step 4: Bonus: Whitening Metals

This battery has a bleaching effect on their coins, as you can see as it became a currency of my country after three minutes.
Do not know if it's a chemical reaction with the acid or an effect of electroplating.
The longer, the clearer the coin.