Riddle Me This

Whenever a Sphinx you control deals combat
damage to a player, you may look at the top two
cards of your library. Separate them into a face
up pile and a face down pile. An opponent
chooses one of those piles. Put that pile into your
hand and the other into your graveyard.
"What is the creature that walks on four legs in the morning, two legs at noon and three in the evening?"