Liquid motion lamps (which most people know as "lava lamps") have been around for decades. The theory behind a liquid motion lamp goes something like this: In the lamp you have two liquids which are: Very close in density; Insoluble in one another. read more
The trick behind a lava lamp is that the waxy compound has almost exactly the same density as the water inside the lamp, and this density changes with temperature. When the wax is cool, it is slightly more dense than the water and it sinks. read more