At plain sight it is a standard black mug as they come a dime a dozen and in big white letters it say COLD. Nothing fancy, but once you add the hot beverage of your choice, the mug changes color and is turning white and in big black letters it states: HOT.
As your drink cools down, it slowly reverts back to black, giving you ample warning to not take that last, dreaded cold sip. The color changes due to heat-sensitive pigment.