You may be thinking of the "Sphinx Riddle".
There was a single Sphinx in Greek mythology, a unique demon of destruction and bad luck, according to Hesiod a daughter of Echidna and of Orthrus or, according to others, of Typhon and Echidna - all of these chthonic figures. She was represented in vase-painting and bas-reliefs most often seated upright rather than recumbent, as a winged lion with a woman's head; or she was a woman with the paws, claws and breasts of a lion, a serpent's tail and eagle wings. Hera or Ares sent the Sphinx from her Ethiopian homeland (the Greeks remembered the Sphinx's foreign origin) to Thebes where, in Sophocles Oedipus Tyrannus, she asks all passersby history's most famous riddle: "Which creature in the morning goes on four feet, at noon on two, and in the evening upon three?" She strangled anyone unable to answer. (The word "sphinx" comes from the Greek Σφιγξ - Sphigx, apparently from the verb σφιγγω - sphiggo, meaning "to strangle" (note that the ng and nx sounds were written in ancient Greek as a double gammas.) This may be her proper name, but The Penguin Dictionary of Classical Mythology states that her given name was Φιξ - Phix.) Oedipus solved the riddle: man - he crawls on all fours as a baby, then walks on two feet as an adult, and walks with a cane in old age. Bested at last, the Sphinx then threw herself from her high rock and died. An alternative version tells that she devoured herself. The exact riddle asked by the Sphinx was not specified by early tellers of the story and was not standardized as the one given above until much later in Greek history. Thus Oedipus can be recognized as a liminal or "threshold" figure, helping effect the transition between the old religious practices, represented by the Sphinx, and new, Olympian ones. http://www.answers.com/the+sphinx+riddle?cat=technology