What was the riddle of the sphinx?

The city of Thebes has been ravaged by a deadly plague. It was caused by the Sphinx, a strange hybrid creature with the wings of a bird, the body of a lion, and the head, neck, and breasts of a woman. She sits on top of a tall pillar, where she asks passersby to answer a seemingly unanswerable riddle: "Who crawls on all fours as a baby, then walks on two feet as an adult, and then uses a walking stick in old age?" If someone answers the riddle correctly, the plague will be lifted.
Unfortunately, no one can, so the plague continues to wreak havoc on the poor, benighted city—until Oedipus steps up to the mark. He successfully solves the riddle. The answer to the question is, of course, man.
Now that the riddle has been solved, the plague immediately lifts. Oedipus, now a hero in Thebes, accedes to the throne, taking over from King Laius, who was recently murdered by an as yet unknown individual.