Like a groggy, hung-over writer, this calculator operates entirely powered by just a few sips of water. I'm not sure exactly what the trick is or why more gadgets don't use the very same technique, but it's particularly cool (not quite as cool as a potato-powered clock, but then what is?) RealGoods.com has the calculator or a simple LCD clock for just $14.
The water isn't the source of the power. The electricity is being produced by the difference in electrode potentials of the anode and the cathode which are inserted into the water (or potato). It's just a battery, it looks a little bit different, but the clock draws a very small amount of current anyway. So in other words: it is powered by a dissolving hunk of zinc.