A substance is said to exhibit thermochromism if it changes colour according to temperature. The most popular example of this is mood rings, the hot fashion item of the 70s. These pieces of jewelry contain liquid crystals (the same liquid crystals responsible for your LCD TVs) sealed underneath a (usually fake) gemstone.
As the temperature increases the liquid crystals go through several phases, in which the crystals align themselves in different ways. These different alignments cause light to interact differently with the crystals, making them appear different colours to the ring-wearer.
Mood rings may have gone out of style, but thermochromic liquid crystals are still used in a few ways. Liquid crystal thermometers are essentially strips you place against your forehead. After about 15 seconds one of the boxes on the thermometer will have changed colour, indicating your current body temperature.
Since they don’t need to be inserted into a bum or placed under a tongue, liquid crystal thermometers are often used for babies, as well as for monitoring the temperatures of fish tanks or reptile habitats and home beer brewing systems.