We are accustomed as we have seen many electronic devices that run on solar energy, including some watches and even cars. But there are even devices that feed on much cleaner or natural if possible, even healthy in many other aspects energies.
For example, since there is a clock that feeds the citric acid of lemon.
The clock is made with a zinc-copper plugs where lemon halves are placed. Acid lemon juice in contact with the zinc and copper electrolysis created, which powers the clock.
Run a watch with a lemon for a week sounds about right. The downside is that changing the lemon, you must adjust the hour and minute. It is also important to have a good assortment of lemons to go feed the clock, but if you have room for a lemon (even a pot) the better and more economical and environmentally friendly.