When the coffee cup is cold it's black and reads "OFF", when a hot liquid is poured inside, it turns white and reads "ON". Pretty clever, huh? I'm not impressed either. But my girlfriend went bonkers over this, claiming it's the best invention of her lifetime (she's dumb as shit). If you want one they cost $25 from Charles and Marie. If you don't want one then I have a lot more respect for you. I still have to get one for my girlfriend though, or she'll throw a goddamn hissy-fit. I'm just hoping the heat sensitive pigment responsible for the color change rubs off and kills her.