The following is from a work-in-progress called The Mahabharata: a Book Report, in which I'm slowly reading through the Hindu epic poem The Mahabharata, and writing a book report on what I read.
Once upon a time, the gods wanted to obtain amrta (or, soma), the nectar of immortality. Vishnu told his fellow gods how to get it--they must churn the ocean. So a coalition of gods and demons uprooted Mount Mandara to use as a churning rod. They used the serpent king Vasuki as a churning rope. Then they began churning the ocean--turning it into milk, then something called ghee, then the ocean birthed the sun, the moon, the goddesses Sri and Sura, the horse Uccaihsravas, a gem called kaustubha, and finally the precious amrta!
The demons tried to sieze this nectar of immortality, but Vinshu intervened and only allowed the gods to drink it. One demon named Rahu got a sip, but Vishnu beheaded him. Today, it is Rahu's head that swallows the sun and moon during eclipses. There was a great war between gods and demons over the amrta, which the gods ultimately won.