Read the excerpt from “From Barter to Bitcoin.” Three friends have just spent an hour gathering colorful rocks at the beach. They arrange their finds on a beach towel and get ready to trade.

Megan sees that Pedro has a bright blue rock. Blue is her favorite color.

“How many of my shiny pink stones do you want for that bright blue rock?” Megan asks Pedro. She has plenty of shiny pink stones, but not a single blue one.

Pedro thinks for a moment and then says, “Five pink stones.”

“Five?” Megan says. “That is a lot. Will you take three?”

Sneha interrupts. She also loves blue. “I will give you this dark green rock for that blue one,” Sneha tells Pedro. Sneha knows that Pedro’s favorite color is green.

“Sold!” Pedro exclaims.

“Rats!” Megan grumbles.

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