Aetiological myths (from the Greek aetion meaning ‘reason’) explain why a certain thing is the way it is or how it came to be. This type of myth is usually defined as an origin story. For example, in Egyptian mythology the sycamore tree looks the way it does because it is home to the goddess Hathor, the Lady of the Sycamore. In Norse mythology, thunder is recognized as Thor's chariot racing across the heavens. Aetiological myths can offer explanations for why the world is the way it is – as in the story from Greek mythology of Pandora's Box which explains how evil and suffering was released into the world – or how a certain institution came to be – as in the Chinese myth of the goddess Nuwa who kept creating human beings over and over and over until she grew tired and instituted the practice of marriage so humans could reproduce themselves. Characters in myths always serve a definite purpose whether they are explaining marriage or an epic mission or decisive battle.
One of the best-known aetiological myths comes from Greece in the form of the tale of Demeter, goddess of grain and the harvest, and her daughter Persephone who became Queen of the Dead. In this story, Persephone is kidnapped by Hades, god of the underworld, and brought down to his dark realm. Demeter searches desperately everywhere for the maiden but cannot find her. During this time of Demeter's sorrow, the crops fail and people starve and the gods are not given their due. Zeus, king of the gods, orders Hades to restore Persephone to her mother and Hades obliges but, because Persephone has eaten a certain number of pomegranate seeds while in the underworld, she has to spend half the year below the earth but could enjoy the other half with her mother in the world above.
This story explained the changes of the seasons in Greece. When it was warm and the fields were bountiful, Persephone was with her mother and Demeter was happy and causes the world to bloom; in the cold and rainy season, when Persephone was below the earth with Hades as his queen, Demeter mourned and the land was barren. Since, in the course of the tale, Demeter teaches the people of Eleusis the secrets of agriculture, the myth would also serve to explain how people first learned to cultivate the earth and, further, as she also teaches them the correct way of recognizing and worshiping her, proper veneration of the gods.