The Riddle of the Sphinx

de bene esse: literally, of well-being, morally acceptable but subject to future validation or exception

According to Greek mythology the Sphinx sat outside of Thebes and asked this riddle of all travelers who passed by. If the traveler failed to solve the riddle, then the Sphinx killed him/her. And if the traveler answered the riddle correctly, then the Sphinx would destroy herself.

The riddle:
What goes on four legs in the morning, on two legs at noon, and on three legs in the evening?