A mood ring is a ring. The ring contains a thermochromic element. An example of a thermochromic element is a liquid crystal.The ring changes color in response to the body temperature of its wearer. The thermochromic element will change the wavelength of light when it is at different temperatures, which is what causes the color change. The propaganda behind being a mood identifier is that it is said that the body will change temperatures to match the mood it is in. when the body is mad it gets hot, when it is sad it gets cold. Now as you could probably guess, this is all just propaganda brought on to sell a cool color changing ring, the body doesn't actually get hot when it is mad or cold when sad. the term thermochromic means that it has the ability to change color because of a change in temperature. Thermochromism is one of several types of chromism. Liquid crystals and leuco dyes are two of the ways people create thermochromism. Liquid crystals are used in precision applications because they can change at more accurate temperatures, but their color range is limited by their principle of operation. Leuco dyes have a wider range of colors but their response temperatures are more difficult to set with procition. The mood ring is actually a brilliant way to advertise this cheap trick. there are also tee shirts made with the dies that can change when you are wearing them. you can find pictures where someone has used a hair dryer to change the color on a shirt and another picture where the person has left his or her hand on the shirt and it changed the color. the color changing shirts are all very cool and are relativly hard to find. there are color changing shirts that use the solar rays to change and reveal colors but they are not quite the same. To recap the mood ring is filled with or a clear jem lays on a layer of a thermochromic element that changes colors with temperature change. Being that the body's skin temperature does not change depending on your mood, the thermochromic "mood" identifier is not worth the investment, however, this is a cool little chemistry experiment to own since it only costs a dollar but to determine your mood you will have more luck going to a shrink or just thinking about how you feel as opposed to relying on a ring and identification card to tell you how you are feeling at the current time.