Wait a minute - I've heard this one before. I think it has to do with the different stages of the human life cycle - you crawl on four feet as a baby in the "morning" cycle, "Two feet" at the noon cycle as an adult and "three feet" as an elderly person with a cane in the "evening" cycle of life. To be honest, I never knew it was the riddle of the sphinx - I just liked the riddle.