Sphinx

Sphinx: Guarding the 'Hourglass of Eurythion' in front of her, sitting straight & tall on a huge stone-bed pedestal on her giant cat haunches on all fours, seeing a normal traveler approaching, she stares down at them for a long moment of a bored, calm, collective blank expression of uninterest before speaking in a Valley-Girl tone. Riddle me this. The person who makes it has no need of it; the person who buys it has no use for it. The person who uses it can neither see nor feel it. What am I?