Literally every fable ends with a good lesson, how to pick! The salt merchant’s donkey sticks out as a memorable one because it’s complex–the donkey’s mistake is not trying to lighten its load, it is doing so excessively, trying to reduce its load to zero every day. The merchant’s revenge seems somewhat cruel, but isn’t actually forced upon the donkey until/unless, even though the sponges are very light on the last day, the donkey tries to make them even lighter by slipping into the water. It’s a classic “pigs get fat, hogs get slaughtered” type message, but with some interesting nuance to it.