The Riddle

I am not too sure why Sphinxes are popular rooftop ornaments round Edinburgh. This is one of a set in Parliament Square. It could be in reference to the use of Sphinxes in Egypt as guardians of tombs and suchlike although this particular creature is Greek and therefore much more likely to kill and eat you if you cannot answer her riddle, making any subsequent need for a tomb unlikely.

This is the riddle, answered by Oedipus who thus rendered the Sphinx extinct as she threw herself from her rock to her death after being defeated.

Which creature in the morning goes on four legs, at mid-day on two, and in the evening upon three, and the more legs it has, the weaker it be?