The Lion, the Wolf, and the Fox explained simply

A Lion, infirm with age, lay sick in his den, and all the beasts of the forest came to inquire after his health with the exception of the Fox. The Wolf thought this was a good opportunity for paying off old scores against the Fox, so he called the attention of the Lion to his absence, and said, "You see, sire, that we have all come to see how you are except the Fox, who hasn't come near you, and doesn't care whether you are well or ill." Just then the Fox came in and heard the last words of the Wolf. The Lion roared at him in deep displeasure, but he begged to be allowed to explain his absence, and said, "Not one of them cares for you so much as I, sire, for all the time I have been going round to the doctors and trying to find a cure for your illness." "And may I ask if you have found one?" said the Lion. "I have, sire," said the Fox, "and it is this: you must flay a Wolf and wrap yourself in his skin while it is still warm." The Lion accordingly turned to the Wolf and struck him dead with one blow of his paw, in order to try the Fox's prescription; but the Fox laughed and said to himself, "That's what comes of stirring up ill-will."