When we look at any object, the wavelength of light reflected by the material determines the color we see. So how is this ring able to change colors depending on its wearer’s mood (Fig. 1)? The magic lies in the shapeshifting behavior of long molecular chains known as polymers!

The polymers in mood rings belong to a special class of materials called liquid crystals—an "in-between" state of matter with some liquid-like and some solid-like properties. More specifically, the long molecules in a liquid crystal polymer can move and jumble around like the molecules in a liquid do, but they can also stack into orderly layers like the atoms in a solid crystal (Fig. 2).

As in other crystals, the wavelength reflected by a liquid crystal depends on the spacing of these layers—a process called crystal diffraction, shown in Fig. 3. Unlike other crystals, however, the spacing of layers in a liquid crystal can change as the molecules move. The specific type of liquid crystal is mood rings is called a thermochromic liquid crystal, or TLC. "Thermo" means heat, and "chromo" means color, so it won't surprise you to learn that TLCs change color in response to temperature. Knowing how temperature affects the structure of liquid crystal polymers—in particular the space between crystal layers—is the key to understanding how mood rings change color.

You might expect liquid crystals to behave more like liquids at warmer temperatures and more like solids at cooler temperatures . . . and you'd be on to something there! But the reality is even weirder and more wonderful.

In the cholesteric phase (so named because some forms of cholesterol exhibit this behavior!), liquid crystal molecules are helix-shaped, like strands of DNA or the threads of a screw. What do you notice about the two screws shown in Fig. 4? They differ in a property called pitch, which you can think of as the "steepness" of the helix, or the vertical distance it takes to complete one full turn. The molecules in a cholesteric liquid crystal also exhibit this property, but they're more like a spring or Slinky than a screw, since their pitch can change under different conditions.

Generally, the helix is more loosely held together at higher temperatures—imagine stretching a Slinky—which increases the pitch. At lower temperatures, the helix contracts and the pitch decreases. As the pitch of individual molecules in a liquid crystal increases or decreases, the layers move farther apart or closer together, respectively (Fig. 5), producing the color change. The whole process is temperature-dependent, with a bit of a catch—the precise relationship between color and temperature depends on the exact composition of the liquid crystal, since impurities in the crystal can also affect the pitch.

Now that you know how it works, do you think a mood ring can really detect your mood? Maybe. The TLC in a mood ring is sensitive to your skin’s temperature, so if you're embarrassed and feel yourself getting warm, it won't just be your cheeks that change color! Increased blood flow to your skin will warm the liquid crystal in the mood ring, and it will change color, too. Then again, the same color change could mean you're flushed with pride or pleasure, and lots of variables besides mood can also affect your skin's temperature. So if a friend's mood ring is black when you're around, don't take it personally—they're probably just cold.