Imagine an ocean covered by a confined atmosphere of water and vapor. When this world is hot enough, the water evaporates; when the vapor cools, it condenses and rains down into the ocean. But if the pressure and heat are such that, as the water expands, the vapor is compressed, eventually the liquid and gas approach the same density. As that critical point nears, something quite extraordinary occurs. Water and vapor no longer remain stable; instead, all through this world, pockets of liquid and vapor begin to flash back and forth between the two phases, from vapor to liquid, from liquid to vapor -- from tiny clusters of molecules to volumes nearly the size of the planet. At this critical point, light of different wave lengths begins reflecting off drops of different sizes -- purple off smaller drops, red off larger ones. Soon, light is bouncing off at every possible wavelength. Every color of the visible spectrum is reflected as if from mother-of-pearl. Such wildly fluctuating phase changes reflect light in what is known as critical opalescence.