Because Yuhwang is element that has strong toxicity, most of birds die after breathing Yuhwang gas even a little. It is hard to find animal that can live after eating Yuhwang, except duck. Especially, duck have strong detoxification ability. By feeding Yuhwang to duck, Yuhwang's toxicity disappears remaining its good components. Duck has great resistance in disease and ability of adapt. So it was used in invigoratant to human since long times ago, because of its good nature of a medicine.