Caoshan's Dharma Body

KOAN:

Caoshan asked Elder De, “’The true Dharma-body of Buddha is like the empty sky. It manifests its form corresponding to things – just like the moon on the water.’ How do you explain the principle of this corresponding?”
De said, “It is like a donkey looking into a well.”
Caoshan said, “You put it in a nice way, but you were able to say only eighty percent.”
De said, “How about you, Master?”
Caoshan said, “It is like a well looking at a donkey.”