The leaf thermometer is a thermometer that represents temperature with colour. When the temperature is between 20C to 25C, the thermometer will be a beautiful green colour. But if the temperature is too cold, it will change to a charred brown colour that's almost black. And if it's too hot, it will gradually change to yellow. Try sticking is on your door or wall. (there is a reusable adhesive seal the back of each leaf).