"To pass," said the Sphinx, "You must first answer my riddle."
"'m not very good at those," said the hero.
"What walks on four legs in the morning --"
"Man?"
"No. Don't interrupt."
"Sorry."
"Right. What walks on four legs in the morning, then says sod it, and goes off to swim for what's looking like the rest of the day?"
The hero stared blankly, wracking his brains. Then, weakly, he guessed. "Is it... a blue whale?"
Sandi the Sphinx grinned widely and stepped back from the door. "You may pass, Mr. Davies."