The Ink N Drink markers are markers that contain heat-sensitive ink that changes color with reducing temperature. So now, coffee shop baristas can not only butcher the spelling of your name with a marker that changes color from green to blue, but they can help you know when it's safe to sip your drink so you don't scald your lip.
The marker's default ink is blue; but when the ink is exposed to surfaces with temperatures exceeding 65 degrees Centigrade (150 degrees Fahrenheit), it changes to green.
The Ink N Drink costs only $4 and works with the paper cups widely used in coffee shops as well as ceramic mugs, depending on the thickness of the material. If a mug is too thick, heat won't be transferred to its outer surface and the Ink N Drink won't be able to carry out its magical color-changing trick.