The markers used by baristas at coffee shops to label cups might soon be useful for more than just identifying your drink. Inspired by the experience of burning your tongue on overly hot coffee, the Ink N Drink markers contain heat-sensitive ink that lets you know which drink is yours, and if it's safe to sip yet.
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]