A Fair Exchange - Lauren Ipsum: A Story About Computer Science and Other Improbable Things - page 52

“Yes?” The teller was a tall, thin man with round glasses. A little card said his name was
Trent Escrow.
“I'm from a foreign country and I want to know what these are worth.”
“Hmm! I've never seen these coins before! And what are these fancy little pieces of paper
supposed to be?”
“That's money, too. One dollar,” Laurie said.
“Money made of paper ,” Trent said. “How strange! They are very pretty, but I'm sorry to say
they are worthless here.”
“Oh no, really? Why?”
“You can't flip a piece of paper fifty-fifty, now can you? But these coins are remarkable,” he
said, examining the poker chip carefully and flipping it a few times.
“Are they worth anything?”
“Well, they look pretty balanced to me. But they aren't certified Fair, so no one will take
them at face value.”
“Tinker said the same thing, but I don't understand,” said Laurie. “Where I come from, you
can flip quarters fifty-fifty, no problem.”
“Ah, but one side might be a little different from the other,” Trent said, “so it might not be
exactly even.”
“Really?”
“Surely. The unfairness shows up better if you spin the coin instead of flipping it. That's how
we test all of our Fair Coins.”
“So my coins are worthless too?”
“Not quite. I can give you an exchange rate: two of your coins for one Fair Coin.”
“Why two to one?” she asked.
“Good question!” Trent said. “Most people don't know this, but it's possible to get absolutely
Fair flips out of even the most unfair coin.”
“I don't believe you.”
“Here, let me show you.” Trent reached into his drawer and handed her a large, heavy coin.
“This is a fake Fair Coin. It looks just like a real one, doesn't it? But it's easy to tell it's fake
because it comes up Heads a lot more than Tails.”
“So how do you get a fair flip?” Laurie asked.