The Regent also smiles. Ah, sphinxes and their riddles. I do appreciate them. Well, she says mock-thoughtfully, as her insubstantial body gradually shifts to a more quadrupedal shape resembling a centaur, though they might not strictly involve the time of day--her shape becomes that of a nymph-like biped--I can certainly think of several answers to that riddle--then becomes a more amorphous, three-legged cloudy mass--though I'm sure none of them would be what you are looking for.
She returns to her natural state and smiles once again. I believe the accepted answer is any variety of humanoid mortal. I appreciate your attempt to bring levity to the conversation, as it has been several years since I have been able to indulge myself with mirth and whimsy in the company of others. However, the regent states solemnly, now is the time for serious concerns, so I would appreciate greatly your saving your riddles until your companions have learned what they wish to know.
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Given that humans obviously aren't the only race in this world by any means, that riddle isn't particularly common relative to any other. The Regent is a spellcaster and a ruler, so she's fairly clever and personable; she could easily figure it out without any need for detect magic, though having her dominion in the part of Faerie that sphinxes hail from probably makes her better with riddles than most other nobles you'd meet.