The lid of Nohot Cup serves as a warning system about the temperature of the drink beneath it. A circular section of heat-sensitive material on the lid expands when it is heated. When the temperature of the drink drops to 30°C, the circle contracts and becomes as flat as the surrounding lid again. If the user tries to drink from the cup when the circle protrudes, it will hit their nose as a warning signal. At that time, the drink is dangerously hot and should not be drunk.