A prince named Oedipus was travelling through the Greek countryside when he neared the city of Thebes. The road to the city was blocked by a monster called the Sphinx.  She had the body of a lion and the head and torso of a woman.  She lay crouched on top of a rock, stopping all travellers to ask them a riddle.

Any traveller who solved the riddle could pass; but those who failed were killed.  When Oedipus approached the Sphinx, no one had yet solved the puzzle. The city had become a prison; its' citizens were afraid to leave, and no one could enter.

But Oedipus was ready for the challenge.  The Sphinx was happy to find another victim.

"What animal," asked the Sphinx, "walks in the morning on four feet, in the afternoon on two feet, and in the evening on three feet?"

Oedipus thought a moment. Then he replied "The answer is Man."

"In childhood he creeps on hands and knees, in manhood he walks upright, and in old age he walks with the aid of a cane."

The Sphinx was so outraged that Oedipus had solved her riddle that she threw herself off the rock to her death.

People of Thebes showed their thanks to Oedipus by making him king.