In old times, people wondered about the changes of seasons, about the stars, the moon and the sun. They could not explain these things, and so they made up 'why' stories about them. The ancient Greeks told about Demeter, the goddess of agriculture, who kept the earth all green all year. One day her daughter Persephone was kidnapped(绑架) by Hades, the god of the dead. Demeter was so unhappy that she let everything on earth die. At last, Hades permitted Persephone to return to her mother for part of the year. During that period, everything on earth grew well again. One season, however, Persephone had to stay with Hades. During that season, the earth remained cold and bare. So winter was explained. American Indians told of an old woman who stood on a high mountain. When the moon was full, she cut off bits of it and threw these bits about the sky. This explained where the stars came from, and why the moon became small. Hundreds of stories like these were passed down by word of mouth. Later, these stories were put in books. Today we read them not for explanation of natural wonders but for enjoyment. The ancient Greeks explained ______.