Aesop Fables: 狼来了 - The Boy Who Cried Wolf
One of my favourite Aesop Fable.
The moral stated at the end of the Greek version is, "this shows how liars are rewarded: even if they tell the truth, no one believes them".
A simple story with a powerful moral that you can use as an example when teaching your children about the detriment of telling porkies. This story is about a shepherd boy who repeatedly tricks nearby villagers into thinking wolves are attacking his flock. When a wolf actually does appear and the boy again calls for help, the villagers believe that it is another false alarm and the sheep are eaten.