The Gods and demi Gods and the devtas, who were all quite taken up with the building of this new habitat, decided to churn the ocean to get out all that lay underneath its hidden depths for the use of the species which were destined to inhabit this planet.
To churn the ocean they needed a big staff and a strong and long rope. Vishnu suggested that the great mountain ‘Mandara’ should be the churning staff and ‘Vasuki’ the serpent should be prevailed upon to become the rope.
The mountain was very heavy and only Vishnu was able to lift it and set it on the ocean bed, because of its weight the mountain started to sink into the ocean floor. Narayan then took the form of a huge tortoise and took the weight of the mountain on his back which was a strong shield making it possible for the ocean to be churned.
Now the real interest of both the devtas and asurs was to get the amrit from the ocean and drink it. This nectar would make them immortal. That is why they whole-heartedly collaborated with each other. It is said that the asurs are always stronger than the devtas, and the Gods were not at all interested in letting the asurs become immortal! But their strength was needed to churn the mighty ocean; Vishnu told the devtas that he would see that the nectar only went to the devtas.
Of course, it was only a part of the ‘Preserver’ that had taken the form of the tortoise. Vishnu in all His glory came on the side of the devtas. He soon succeeded in his getting the amrit to the devtas, by luring the asurs by transforming Himself into a beautiful woman named Mohini and except for one asur, Rahu, who had seen through the game succeeded in getting a few drops of it and became immortal. He was pointed out by the Sun God and the Moon God and quickly Vishnu cut him in two to try to destroy him, but Rahu had become immortal and could not be destroyed.
Both the parts became different identities and both became immortal and are now known as Rahu and Ketu. Now they trouble the Sun and the Moon by eclipsing them at intervals and the Sun and Moon suffer the humiliation for sometime every year. The people of our world used to take out guns and fire blank shots at them or shout ‘Rahu Ketu ke hattyare. Chandiama or Suraj devta ko chor.’ (Rahu and Ketu you scoundreds leave the Moon or Sun God alone.)