What is the riddle of the Sphinx?

The riddle of the Sphinx is as follows:

“What animal walks on four legs in the morning, two at noon, and three at night?” the Sphinx asks Oedipus, the hero of Sophocles’ play Oedipus Rex (426 B.c.).

Oedipus answers that it is man (crawling as an infant, walking erect as an adult, and walking with a staff in old age).