Potato Clock

A potato clock runs by converting chemical energy into electrical energy, which is then used to power a clock. The potatoes, in combination with zinc and copper strips (which act as electrodes), act as a battery. A galvanised nail provides the zinc electrode, and a copper wire provides the copper electrode. Most people aren't aware that this is possible, which is what makes it so interesting.
The key fact is that the potato is mildly acidic and that provides the ions to "complete the circuit". This is because potatoes contain phosphoric acid (H3PO4) which is a mild acid. The energy required to pwer the clock does not come from the potato; it comes from the chemical change in the zinc when it dissolves inside the acidic content of the potato.
What happens is that the zinc is oxidized inside the potato, exchanging some of its electrons with the potato acid in order to reach a lower energy state, and the energy released provides the electrical power. The presence of the acid, which dissociates, provides positive ions of hydrogen in the solution, which enables the process.
Let's imagine first that we have one potato and the nail and copper wire are inserted into this potato, with a wire connecting the two strips. Make sure that the zinc (nail) and the copper wire do not touch each other inside the potato, otherwise the battery will "short". This potato battery works as follows:
1) The zinc atoms in contact with the potato dissolve in the presence of the acid. This causes some electrons to separate from the zinc atoms. As a result of this, positively charged zinc ions, and negatively charged electrons, are produced:
Zn -> Zn++ +2e- (oxidation at zinc electrode)
2) The electrons produced in the above reaction travel out through the zinc, through the wire, and into the copper strip also inserted in the potato. They do this because they are attracted to the positive hydrogen ions in the potato, located on the copper side (these hydrogen ions are there due to the acid content of the potato).
Since the electrons cannot pass through the potato itself, they pass through the wire joining the zinc and copper strips. These electrons then combine with these positive hydrogen ions (on the copper side) and produce hydrogen gas, which then bubbles away.
2H+ + 2e- -> H2 (gas) (reduction at the copper electrode)
Note that the above chemical reaction happens spontaneously. It is self-driven. The electrons are forced to travel an external path, and if this external path is connected to an electrical device, such as a clock, it powers the device.
Two potatoes can be connected together in order to double the voltage, the same way you join two batteries together to double the voltage. This voltage is sufficient to power the potato clock.
Any acid will do the job. So lemons will work as well, since they are also acidic (they contain citric acid), but the power generated is smaller than with potatoes. You can also try lighting an LED lamp with the potato battery. Anything that needs low currents to operate should work. To make the LED shine brighter, you can use 3 potatoes in series. Or even more! Once the zinc is used up, the battery will stop working.
Note: Make sure you use galvanised nails. In this process, steel is coated with zinc. Otherwise it will not work. Since zinc dissolves from the nail into the potato, do not eat it afterwords.