The first thing Laurie tried to do in Probability was buy a muffin, but her strange money wasn’t any good. Nobody wanted her “Quarter Dollars.” Nobody wanted any algorithms, either.

“Message for you!” A red daemon carrying a mailbag appeared out of nowhere on the street.

“For me?” Laurie said.

“You don’t see me talking to anyone else, do you?”

The daemon handed Laurie an envelope and walked away, and once the note was out of the envelope, Xor made short work of the gibberish in Colonel Trapp’s latest message. This one was shorter and more useful than the last.

After reading the Colonel’s suggestion, Laurie remembered a building she’d walked past earlier: the Fair Coin Savings & Loan. She rushed back there and stepped inside to find it looked just like banks back home.

“Why are we at a bank?” asked Xor.

“Because I’m getting hungry. It’s almost dinner time.”

“Finally. But you can’t eat money!”

“I can’t spend my money either, which means no food. Fair Coins are all anyone takes around here!”

Laurie walked up to the teller window and put everything money-like she had on the counter: a few dollar bills, three quarters, a penny, a dime, and her lucky poker chip.

“Excuse me, sir?”

“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.

“Just flip it twice. If you get Heads-Tails, use Heads as your answer. If it comes up Tails-Heads, use Tails as your answer. If you get Heads-Heads or Tails-Tails, just start over.”

“Oh, I see. The unfairness cancels itself out!”

“Right you are. No matter how unfair a coin is, the odds of getting Heads then Tails will always be exactly the same as getting Tails then Heads.”

Trent Escrow solemnly exchanged Laurie’s five coins and lucky poker chip for three Fair Coins.

“And that’s why the rate is two to one.”

“Thank you!” Laurie rushed outside to catch the baker before he closed. Dinner!