Riddle Of The Sphinx II: Sustained Release Riddlin'

I was driving down to LA when the cops pulled me over. "You have to turn back sir, the Sphinx here eats any traveler who can't answer her riddle."

"I've trained my whole life for this" I said, and stepped on the gas. Soon I saw a Sphinx lounging in the middle of the road. When she spotted me, she asked: "What has braces, crowns, and retainers, but is not teeth?”

"A medieval king in armor. My turn. What has pupils, irises, and whites, but is not an eye?"

"A gardening class during apartheid. How is a river like the Federal Reserve?"

"It maintains liquidity despite rushes on the banks. What has wings, but cannot fly - fins, but cannot swim - and heels, but cannot walk?"

"Helsinki General Hospital." The Sphinx licked her lips. "But tell me, how is Lord Nelson like a cigar?"

"They both have one I and won sea. How is Mary Mary Quite Contrary like the Norse god Odin?"

"They both have one eye and RYs," the Sphinx answered. "You're good at this. But you won't get my next riddle. It's the best riddle ever. I spent five thousand years working on it and it's totally impossible. Are you ready?"

"I was born ready."

"What has four legs in the morning, two legs in the daytime, and three legs in the evening?"

"Hmmmmm," I said. Then it came to me. "A trip from San Francisco to Milan. Here, check out the Kayak.com results." I held up my cell phone:

"Hey," said the Sphinx. "That's a pretty good answer. You wouldn't believe what the lame motherfucker I asked last time tried to pull on me."