Riddle: What walks on four legs in the morning, two legs in the afternoon, and three legs in the evening?
Answer: Man. He crawls on all hands and knees as a baby, walks on two legs as an adult, and walks using a cane in his old age.
This is a well known riddle, cited from The Braingle.com.
According to the UnMuseum, this riddle was given by the Sphinx to all travelers. It would eat all the travelers who could not answer his riddle, until Oedipus gave the correct answer of “Man,” and caused the Sphinx to die.
The fact that his riddle from such a long time ago is so well known even today shows the universality of riddles about humanity. This riddle causes people to reflect and look at themselves and their lives in order to answer the riddle. It makes people think about the concept of life and aging, and brings an awareness to the natural progression of life in a clever way.