So how efficient are they? Well, this formula here where E stands for efficiency or engine efficiency, can basically tell you how efficient an engine is. If you know the temperature of the cold body and the temperature of the hot body, you can find the efficiency. And this also shows you can see from algebra and basic calculus that ends, this becomes zero or tenths of zero. That is, as TC decreases and Th increases or the difference between these two guys increases, the efficiency also increases. Okay?