Motion Lamp
Liquid motion lamps are actually very simple devices. They are basically based on a very simple scientific principle and consist of only a few simple components.
They must have a compound that makes up the floating "blobs", a compound that the blobs float in a lamp that illuminates the display and provides the heat necessary to move the blobs to create the floating blobs, the two compounds in a motion lamp must be immiscible or insoluble. All this means is that liquid A does not dissolve in liquid B and they two do not mix, so you see two separate liquids, one floating on top of or within the other.
The two examples of immiscible compounds are oil and water. If you fill a jar with common mineral oil and water, you will get a water layer with a layer of oil floating above it. The combination of water and oil in a jar has a similar look to a commercial motion lamp with its light turned off; in a cold lamp you see two separate layers.
The best thing about motion lamps, of course, is that they produce distinct amorphous blobs that rise and fall in the lamp's "globe" on their own. In order to produce this effect, you need to pick your two insoluble compounds very carefully. In the oil and water jar, the water ends up on the bottom because it has a much higher density than oil. We can simply say that a liquid with a higher density pushes a lower-density liquid upward.
If you take a look inside a motion lamp when it is turned off, you will find a solid waxy compound on the bottom of the globe. This solid compound is only a little denser than its surrounding liquid compound. When you turn on the light at the base of the globe, here is what happens the solid quickly turns into a liquid and expands, giving it a lower density than the surrounding liquid and a warm blob is now slightly less dense than the surrounding liquid, so it rises to the top of the globe. Because it is farther away from the heat source, the blob cools slightly, becoming denser than the surrounding liquid.
The blob then sinks to the bottom of the globe, where it heats up enough to rise again.
This is a pretty simple idea, but it is actually fairly complicated to balance all the elements -- the compounds, the heat source and the size of the globe so that the blobs are constantly moving around. In fact, the companies that produce commercial motion lamps guard their ingredients very closely, and motion lamp enthusiasts have had a very hard time reproducing the displays you see in the commercial models.