Once upon a time in a usual morning the king of a far country went for a walk in his own garden. Soon after he returned to his throne listen to envoy's message. But the envoy said nothing - he just advanced to the throne where the king was awaiting him, and stooping down, traced on the floor with a rod which he held in his hand a black circle. The king and his courtiers were equally mystified, but the envoy was calm. After that, the king called all ministers to solve this problem. Nobody had a good idea, and the king said:"Unless before sunset you could find someone capable of solving the mystery, I would hang you all!" Could anybody solve this mystery?