The riddle we all know is- What creature walks on 4 legs in the morning, 2 in the afternoon and 3 in the evening? The answer is a human in relation to the stages of life.
The second, and possibly older, riddle it thus- There exists two sisters. The first gives birth to the second and she in turn gives birth to the first. Who and two sisters? The answer is Day and Night in relation to the rising and setting of the sun.
It is (modernly) explained the he arrived at the answer because of his own lameness (his father tethered and pirced his legs together as an infant, making him unable to crawl). Or he is very observant of humans.
But what explanation could explain how Oedipus got his answer about the second riddle I mentioned?