One day a small rat surfaced from his nest to find himself between the paws of a huge sleeping lion, which immediately awoke and seized the rat. The rat pleaded with the fierce beast to be set free, and the lion, being very noble and wise, and in no need of such small prey, agreed to let the relieved rat go on his way.
Some days later in the same part of the forest, a hunter had laid a trap for the lion, and it duly caught him, so that the lion was trussed up in a strong net, helpless, with nothing to do than wait for the hunter to return.
But it was the rat who came along next, and seeing the lion in need of help, promptly set about biting and gnawing through the net, which soon began to unravel, setting the great lion free.
The moral of the story is of course to make the world your debtor – even the humblest of folk may one day be of use.