Subject: Water used as a power source

I have seen a "Skilcraft" kit which contained two reservoirs (Fire 
Hydrants) which power a clock.  These reservoirs are filled with nothing 
but tap water.  Two wires come from the clock one copper to one reservior 
and the other zinc or steel which leads to the other reservior.  A jumper 
wire runs from reservior to reserivor with the opposite metal probe.

         Zinc Jumper     Jumper wire  Copper Jumper

          Tap Water                     Tap Water

       Copper to clock     Clock       Zinc from clock

What is the principal behind this?

Re: Water used as a power source