The famous riddles of the sphinx: "What goes on four legs in the morning, two legs in the afternoon, and three legs in the evening?" Man, who crawls on all fours as a baby, walks on two legs as an adult, and uses a walking cane when old. "There are two sisters, one gives birth to the other, and the other also gives birth to the first. Who are these sisters?" Day and night.
These are the riddles of the Greek sphinx, usually depicted as a beast with a woman's head, a lion's body, and eagle's wings. The Greek sphinx was seen as a monster, devouring travellers who were unable to answer its riddles. In a Greek story, the riddles were answered by the infamous Oedipus, causing the sphinx to implode or something. You know, classic Greek mythology.
The Egyptian sphinx was different, however, that it had the head of a man instead of a woman. It was not an evil monster but instead guarded temples and tombs of the pharaohs. An example would be the Great Sphinx of Giza. Although the Egyptian sphinx would likely be the one to come to mind first, it has little to do with the equally famous riddles of the Greek sphinx.