A lava lamp (or Astro lamp) is a novelty item used more for decoration than illumination—the slow, interesting rise and fall of variously-shaped blobs of wax is suggestive of lava, hence the name. The lamps are available with a variety of styles and colors of wax and liquid.
The lamp contains a standard incandescent bulb or halogen lamp which heats a tall (often tapered) glass bottle containing water (often with glycerol derived additive) and a transparent, translucent or opaque mix of wax and carbon tetrachloride (although other combinations may be used). The wax is slightly denser than water at room temperature but is less dense under warmer conditions. This occurs because wax expands more than water when both are heated. When heated, the wax becomes fluid, its specific gravity decreases, and blobs of wax ascend to the top of the device where they cool and then descend. A metallic wire coil in the base of the bottle acts as a surface tension breaker to recombine the cooled blobs of wax after they descend.
The bulb is normally about 25 to 40 watts. It may take 30 to 60 minutes for the wax to warm up enough to freely form rising blobs (depending on the original temperature).
Once the wax is molten the lamp should not be shaken or knocked over or the two fluids may emulsify and the wax/blobs will remain cloudy rather than clear. The only means to recombine the cloud of wax is to turn off the lamp and wait a few hours for the wax to settle back down at the bottom, forming one blob once again.