Folktales of every culture are often "etiological" -- concerned with the origins of how things are in the world.   Why does the moon go through phases?  Where did animals come from?  Why do we have to die?   Rudyard Kipling assembled a very famous collection of etiological folktales for children in a book he called Just-So Stories (1902).   Here is an example of an etiological folktale from a different source, probably also for children:

WHY DOES THE BAT FLY AT DUSK?

      In the beginning times the birds and animals fought a war.  The bat was confused and did not know which side to take.  When the birds seemed to be winning, he tried to join them. The birds asked what he was doing on their side, he said "look at my wings; I am one of you." After much argument the birds accepted him as one of their own.  But then the animals began to win.  So the bat tried to join the animals.  "See my teeth?  I am one of you."  The animals would not accept him, and now neither would the birds.  From that day on the bat has been such an outcast that he cannot go out except alone with his own kind in the dim light of dusk.  [Versions of this are told by both the Yoruba and the Ibo of Nigeria.]