The myth of Rahu and Ketu

Originally, there was only Rahu and Ketu was not existed in ancient time.

Once upon the time. when the earth was in darkness, demons and gods pulled on either side. They were in a terrible war over the pot of immortal drink at the bottom of the Milk Ocean. Brihaspati advised the gods to take help of the demons to churn the oceans to find Amrita (the the drink of immortality).
There was a demon named Swarbhanu (Rahu) and secretly drank Amrita. Surya and Chandra gods saw him to do it, told Vishnum and Vishnum cut/chopped Rahu into two.
Rahu got a head with the body of snake and Ketu got the head of snake with the human body. They both are immortal because they drank Amrita.

Rahu opens our desire and Ketu leads us into conflict.