[Caption - Low-boiling liquid, such as freon or propane, fills one bottle of each pair. The opposite bottle is empty and void of air. The liquid collects in the lower bottle, which is immersed in warm (solar-heated) water. Heat from the water (or a solar reflector, or any other source slightly warmer than the surrounding air) vaporizes the liquid and forces part of it up through the connecting tube and into the empty bottle on top. Gravity does the rest: The heavy bottle starts down; the lighter bottle floats up. As each pair shuttles its liquid mass back and forth, the whole thing turns and repeats the process endlessly.]