The marker writes in blue ink, but when it's exposed to temperatures exceeding 150 degrees Fahrenheit—like from a hot beverage inside a paper cup—it changes to green, letting you know your drink is too hot.

When it's cooled sufficiently, though, the ink changes back to blue so you know it's finally safe to chug without scalding your mouth. The $4 marker of course works on the paper cups used in popular coffee shops, but it can also work on ceramic mugs too, depending on how thick the material is. If the heat from the drink isn't effectively transferred to the outside of the cup, the ink won't be able to do it's job. [Ink N Drink]