A man (or a human to be politically correct). In Greek mythology, a monster in the city of Thebes called the Sphinx would ask this question to any passerby and if they couldn't answer it correctly, the Sphinx would eat him/her. In the morning, the man is a baby and he crawls on fours. In the afternoon, a man is grown and walks on two feet. In the evening, the man is old and requires a cane to walk. The man who solved this riddle was Oedipus who, upon solving it, caused the Sphinx to fling herself off a cliff and commit suicide.