If you know those two numbers, 0 and 40, you can get a general idea of what the temperature is in Celsius without doing any math. If you hear 20, you know that's a moderate temperature because it's right in between. If you hear 30, you know that's fairly warm. If you hear 10, you know that's chilly but not freezing. Below 0 or above 40 are extreme cold and extreme heat, respectively.