(from the Mahbharata)

The gods came together on Mount Meru to find a way to win the amrita (elixir of immortality). Vishnu suggested: "Let the Devas and the Asuras churn the pail of the ocean, and then the elixir will emerge from the froth, with all herbs and jewels."

Everyone went to Mount Mandara, uprooted it, and set it, as a paddle on the back of the tortoise Kurma, who was Vishnu incarnated. Then they used the snake Vasuki as the rope and began to churn the ocean. The Asuras and Danavas took hold of one end of Vasuki and the Devas the other, twirling Mount Mandara about, so that its trees toppled and the friction set them alight. Indra put out the fire with water from his clouds, but the sap of all the plants flowed into the ocean, and the ocean turned to milk and then to butter.

Continuing their efforts they churned more, and those arose the Sun, the Moon, the goddess of Fortune, and lastly the divine physician Dhanvantari holding the elixir. Vishnu deluded the Asuras into surrending the elixir and gave it to the Devas to drink. However Rahu snatched a drop, but before he could swallow it, Vishnu decapitated hijm. Ever since the moon (the way the amrita manifests itself) waxes and wanes as it disappears and then reappears from Rahu's throat. The Asuras fought but the Devas won, and put Mount Mandara back in its proper place.