The Yaksha asked, ‘What is that which doth not close its eyes while asleep; What is that which doth not move after birth? What is that which is without heart? And what is that which swells with its own impetus?’ Yudhishthira answered, ‘A fish doth not close its eyes while asleep: an egg doth not move after birth: a stone is without heart: and a river swelleth with its own impetus.’

In plain words: The Yaksha asks simple riddles. Yudhishthira answers: a fish sleeps with its eyes open, an egg does not move after it is born, a stone has no heart, and a river grows by its own force.