Lava lampA lava lamp is a lamp which is typically used more for decoration, than illumination.
It consists two parts, the actual lamp, and a container, typically glass containing a liquid, some wax, and a metallic coil. The container is placed on the actual lamp, so that the heat from the lamp causes the metal coil (now resting at the bottom of the container) to heat up.
The wax and the liquid have similar densities, however, the wax has a slightly higher density than the liquid, when the wax is cool, and a slightly lower density than the liquid, when the wax is warm. This is due to the fact that most substances expand when heated, but at different rates.
Wax at the bottom heats up, until the density of the wax is less than that of the liquid, at which time, the wax rises to the top. At the top, the wax cools down, due to the atmosphere, and the density again drops, until the wax again falls. There may be both wax rising and wax falling at the same time. The behaviour of the wax is unpredictable, due to chaotic behaviour, and hence lava lamps have been used to manufacture onetime pads for cryptographic use. .
See also: Plasma lamp