Read the following Greek myth.
Sisyphus and the Boulder
Sisyphus, the king of Corinth, was a clever man known for his trickery. He would steal and lie whenever he could get away with it, sometimes even fooling the gods.
After one too many tricks, the god Zeus decided to punish Sisyphus. Zeus sent Thanatos, the god of death, to tie Sisyphus in chains and take him down to the underworld, the land of the dead. But Sisyphus was up to his old tricks. He asked Thanatos to show him how the chains worked, and then managed to chain Thanatos instead so he could escape.
Angry, Zeus sent another god to take Sisyphus back to the underworld for good. Sisyphus was given a heavy boulder to push up a tall hill. Zeus told Sisyphus that he had to push the giant rock over the top of the hill to the other side.
Using all his might, Sisyphus tried and tried to push the boulder over the top of the hill. Yet each time he reached the top, the boulder would roll back down. He went on pushing the boulder up the hill, over and over again, forever.
What is the main theme or lesson of the myth?