The Fox and the Frog

There was a frog who claimed to be trained in the physician's art, acquainted with all the medicinal plants of the earth, the only creature who could cure the animals' ailments. The fox listened to the frog's announcement and exposed his lies by the colour of his skin. 'How can it be,' said the fox, 'that you are able to cure others of their illnesses, but the signs of sickness can still be seen in your own face?'