What is a liquid crystal thermometer?

Those thermometers use liquid crystals to measure temperature. More specifically, they use chiral nematic liquid crystals--long asymmetric molecules that arrange themselves in orderly spirals in the liquid. When light strikes these spiral structures, some of it reflects. But the reflection is strongest when the light's wavelength is an integer or half integer multiple of the spiral's pitch--the distance between adjacent turns of the spiral. Since light's wavelength is related to its color, the light reflected by these liquid crystals is colored. Because the pitch of a chiral nematic liquid crystal changes with temperature, so does its color. Slightly different liquid crystals are inserted behind each number on the thermometer so that each number becomes colored at a different temperature.

copied from:
http://rabi.phys.virginia.edu/HTW/thermometers_and_thermostats.html