Oedipus had to enter the gates of Thebes. Guarding this gate was a Sphinx, a giant lion with the head of a beautifual woman.
She told him he could either:
Turn away
Answer her riddle and pass
Try to answer the riddle and if incorrect, die
Around were the bodies of many thousands of souls who had tried before to pass.
The riddle was thus (if you know the answer already, do not spoil it; and do not google, if you can not guess it I will give the answer).
Who am I?
In the morning, I walk on four legs
In the afternoon, two legs
And in the evening I use three legs
Who am I?
When this riddle was answered correctly by Oedipus, the sphinx threw herself off the cliff.