This riddle was first used in Oedipus Rex (?) by Sophocles, right? Oedipus has unknowingly killed his father on the road, and has arrived at Thebes. He meets the Sphinx, who is terrorizing the city, and answers the riddle correctly. The Sphinx kills herself, and Oedipus becomes a hero. He marries the widowed queen, who turns out to be his mother.