Or rather, for mothers in developing nations who don't have ready access to nurses and traditional mercury thermometers [d'oh, mercury?!!]. The Thermospot was designed by one John Zeal in response to the apparently widespread infant hypothermia problem in many cultures of Africa, southeast Asia, Nepal, what have you.
The Thermospot sticks on a baby like a decal on a banana, and mothers are told to watch that the smiley face doesn't disappear. As long as it's there, the kid's body temperature is warm enough. But if the smile disappears, and the face turns black, that's the sign there's trouble, and the mother needs to wrap the baby and warm him up.
Let me repeat that for sheer ironic impact: when you see the black face, that's when you know there's trouble. Black face=trouble. Simple as that. Any questions?