The riddle the Sphinx spoke to Oedipus was, "What has one voice and is four-footed, two-footed and three-footed?" When Oedipus gave the correct answer, "man," the Sphinx threw herself off a cliff and died.
According to the Encyclopedia Britannica, Oedipus answered "man" because he "crawls on all fours in infancy, walks on two feet when grown and leans on a staff in old age." In the legend, the Sphinx waited at the entrance to the city of Thebes and asked the riddle of travelers. Because none of them could answer, they were strangled and eaten. In early versions of Oedipus' legend, written during the time of Homer in the eighth century B.C., the Sphinx is absent. It appears soon after in a sixth-century version by Hesiod and later in Sophocles' three Theban plays written in the fifth century B.C.