What is that which has one voice and yet becomes four-footed and two-footed and three-footed?
So the Sphinx asked many times, devouring those who gave false answers, until Oedipus came along and said: Man.
The riddle relies on singling out a few properties (footedness) of its answer (man). The air of mystery is removed further, if you see the answer and riddle presented together in a more standard format:
Man, four-footed at sunrise, two-footed at noon, three-footed at sunset.
This sentence (fragment) is now a metaphorical description qualifying the familiar in less familiar terms.
(You can use this principle to make riddles of you own. Take a metaphorical description, remove the familiar thing being described and pose the rest as a question. For example, what first smells of breakfast, then later smells of hell?)
A Quiver Of Quotes 