The Pond could not see over the hill.
The market was in a dip by the water, and the next village was an hour away on the other side, and nobody in the square had the faintest idea what anybody over there was paying for anything.
So there was a lookout.
Christina the rabbit sat on the top of the hill all day where she could see both markets at once, and every so often she stood up and shouted down what the next village was paying.
"EIGHT SHELLS A SACK."
And the whole square would hear it, and the barrels and the scales and the stalls and the library would all shift a little, because now they knew.
She had been doing it for nine years. Bao had grown up hearing her.
"What if she is wrong?" said Bao.
"Then everybody is wrong," said Moolah.
"Everybody?"
"Every price down here comes off that hill. You cannot see over it and neither can anybody else." Moolah pointed at the square with one paw. "All of this is downstream of a rabbit."
On a hot Thursday in August the rabbit fell asleep.
She had shouted eight shells a sack at eleven o'clock, and then she had put her head down in the grass, and over the hill the next village had a cart break an axle and their whole sack price went to fourteen by noon.
The Pond traded at eight all afternoon.
Animals came over the hill at four o'clock, bought everything in the square at eight shells, and walked it straight back over the hill.
Nobody in the Pond had done anything wrong. The barrels had worked. The scales had worked. The book keepers wrote everything down correctly and read it all back at sundown and every single line matched.
The square had simply spent an afternoon acting on a shout from eleven o'clock.
Chrisitna was very sorry. She is still on the lookout, because somebody has to be.
But there are three lookouts on the hill now, and they shout together, and if one of them says something the other two do not, nobody down in the square moves at all.
Margin Note: Everything a market knows about the outside world, somebody carried in.
The Honest Bit: The lookout is an oracle. A chain can see everything that happens on itself and absolutely nothing that happens anywhere else, so prices from the outside world have to be carried in by somebody. If what they carry in is wrong, everything built on top of it is wrong, and nothing else has to break for that to happen. This is why serious systems use several lookouts instead of one, and why "what is the price" is a much harder question than it sounds.
The Question of the Day: On that Thursday every machine in the square worked perfectly. Say what went wrong anyway, without using the word rabbit.
Try It: Find out which oracle a lending market on BNB Chain uses for its prices. Then find out how many sources it takes before it believes a number.
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