Or any other fluid for that matter, it just needs to be in a liquid state. As an environmentalist to the core, I just love seeing gadgets which are powered by alternate sources of energy - take this Coke powered clock for instance. It has an internal converter which extracts electrons from water (or other liquid) molecules and provides a steady stream of electrical current acting as a fuel cell to generate power to the clock.

Costing a mere $12.99 , Thingeek says you oculd even put in flowers (thats if you decided to use water, somehow those crazy flowers just dont like Coke). It goes on and on, so unless you have some really thirsty flowers, you need not even worry about refills.