You can actually séé the temperature of glowing things because of something called Black Body Radiation. Every single solid material gives off a certain color of light at a certain temperature. Sadly the theory doesn't perfectly match what our eyes see in the real world. That's why I made this Mug! These color bands more closely resemble the colors we perceive when a material is at these temperatures. Ambient light, the color of the material and its transparancy do still change a material's appearance a bit. So take that into account.
The mug has Celsius on one side and Fahrenheit on the other.