The hammer rang once. A spark jumped off the anvil and did not go out. It waited. They followed.

The Reading Wall went up in the school entrance in September, and by October it was the most looked-at thing in the building.

It was a huge board, floor to ceiling, painted like a mountain. Each class had a little climber cut out of card, and every Friday the climbers moved up the mountain according to how many minutes of reading the class had logged that week. At the top of the mountain was a flag, and beside the flag a sign: First class to the summit wins a trip to the city. Every student had a reading log on the school app. You tapped a button when you started reading and tapped it again when you stopped, and the minutes went into your class's total.

Tara volunteered in the library on Wednesday afternoons, shelving returns and helping the younger students find books. She had liked the wall at first. It was cheerful. It made reading look like something people did on purpose, which at their school it often was not.

Mrs. Adeyemi, the librarian, had liked it even more. She had worked in that library for twenty-two years, mostly alone, and for most of those years the most popular thing in the room had been the radiator. Now, suddenly, children came in and asked her things. On the first Friday in October she stood by the wall with her arms folded and her face glowing, and said to Tara, "Do you know what the average was last year? Ninety minutes a week. Logged. This week it's four hundred and ten."

"That's amazing," said Tara.

"It is," said Mrs. Adeyemi. "It really is."

At the end of October the head teacher, Dr. Hale, announced in assembly that the Reading Wall had been the most successful thing the school had tried in years. Minutes logged had more than quadrupled. The governors were delighted. Next year there would be a Maths Wall and a Science Wall as well.

Tara clapped with everyone else. But on the way out of assembly she heard two boys from Year Nine laughing about something, and one of them said, "Mine ran all night again. Six hours. Absolute legend," and the other one said, "You're going to get us to the summit single-handed."

She did not know what they meant. Not yet.

By the middle of November, everyone knew what they meant.

It came out in pieces. A Year Eight girl told her tutor that she logged minutes on the bus every morning by opening a book on her lap and looking out of the window. A boy in Year Ten admitted, quite proudly, that he started an audiobook on his phone at bedtime and let it run until morning. "It's reading," he said. "Technically. The book's being read." Someone else had found that if you tapped the button and then left your phone in your locker, nobody could tell. Whole classes had started reading the thinnest books in the library, because you logged the same minutes but felt better about finishing.

The staff room was not amused. At the Wednesday meeting the answer seemed obvious to almost everybody.

"More rules," said Mr. Whitlow, the deputy head. "Photo proof of the page. Page numbers at the start and end. A parent's signature each week. We tighten it up."

Heads nodded. It sounded sensible. It sounded like what you do when people cheat.

Tara was there, because the meeting was in the library and she had been shelving. She had not meant to listen. She listened anyway.

Boaz was there too. He was in her year and moderated the school's online forum, which meant he spent a lot of his life watching people find their way round rules. He had come in to return a book and stayed.

"Can I say something?" said Boaz. Mr. Whitlow looked surprised but nodded.

"I moderate the forum," said Boaz. "Every time we've added a rule to stop people gaming the upvotes, they've found a way round it within a week. Every time. A harder rule just gets a cleverer workaround. If you ask for photo proof, someone will photograph the same page every day. If you ask for page numbers, they'll make them up. If you ask for a parent's signature, you'll punish the kids whose parents work nights." He shrugged. "You'll spend all year chasing loopholes. And you'll still have the same problem."

"Which is what?" said Mr. Whitlow, a little sharply.

Boaz hesitated. "I'm not sure yet," he said honestly. "But I don't think it's the loopholes."

On her way home that afternoon, Tara found the boy from Year Ten, the one with the audiobook, sitting on the wall outside the bus stop. His name was Callum. She had shelved books for his class in Year Seven.

"Can I ask you something?" said Tara. "The audiobook thing. Why?"

She expected him to laugh. He didn't.

"Do you know how many of my class have been to the city?" said Callum. "Like, ever? Four. Out of twenty-six. My mum's never been. It's an hour on the train and the train costs more than we spend on food in a day." He picked at the edge of the wall. "The trip's free. If we get to the top, we all go. Everyone. So yeah. I leave it running. Nobody told us the point was to like books. They told us the point was minutes. So we got minutes."

"Do you read?" said Tara. "Any of it?"

Callum was quiet for a moment. "I listened to one properly," he said. "About a lighthouse keeper. It was all right, actually. I fell asleep before the end, though, so I don't know what happened." He looked sideways at her. "Does the library have it?"

It did. Tara told him which shelf.

She walked home thinking that the wall had done three different things at once, and that the cardboard climber could only show one of them.

That evening, at the Whetstone, Tara told the story to Opal.

Opal designed the surveys for the school's student council, and she was the most precise person Tara knew about words. She did not say "basically" or "sort of". If you said "loads", she asked how many.

"So what was the number standing in for?" said Opal.

"Get people reading," said Tara.

"Say it more exactly," said Opal. "In words, before any numbers. What did they actually want?"

Tara thought. Danya, who had come with her, was colouring in the corner of a napkin and pretending not to listen.

"They wanted kids to read books," said Tara slowly. "Books they chose. And to like it enough to come back for another one."

"Good," said Opal. "That's the real thing. Now, can you see that on a wall?"

"Not really."

"No. You can't measure 'liked it enough to come back'. Not easily. So they picked something you can measure, that usually goes along with it: minutes spent reading." Opal drew two circles on a napkin, overlapping. "Most of the time, more minutes means more of the real thing. So minutes is a good stand-in. But a measure is a stand-in, not the thing. Minutes logged isn't reading. It just usually travels with it."

"Usually," said Danya, not looking up.

"Usually," said Opal. "A stand-in usually travels with the thing. Until somebody puts a prize on the stand-in."

Loop was in Year Ten and made videos about how social media apps decided what to show you. He had four thousand followers and an opinion about everything. When Tara described the wall to him on Thursday lunchtime, he laughed out loud.

"Of course that happened," said Loop. "Whatever you reward, you get more of. Reward the clicks and you get clicks. The wall rewarded minutes. So it got minutes." He pulled out his phone. "Did anybody check if it got books?"

Nobody had. So Tara asked Mrs. Adeyemi, who kept the library loan figures in a battered blue folder, the same way she had for twenty-two years.

Mrs. Adeyemi went quiet as she turned the pages.

"Last November," she said, "we lent three hundred and twelve books. This November, two hundred and seventy." She looked up. "That can't be right."

It was right. She checked it twice.

Loop wrote it on the back of a napkin, under the minutes:

Minutes logged: 90 a week → 410 a week.

Books borrowed: 312 a month → 270 a month.

"There it is," said Loop. "They didn't break the rule. They followed it. The rule said minutes. Minutes is what they made." He tapped the napkin. "The loop feeds whatever it measures. It doesn't care what you meant."

Mrs. Adeyemi sat down rather suddenly on the library step-stool.

"But some of them really are reading," she said. "I know they are. I've seen them."

And that was true too. She opened the blue folder again and showed them a list she had been keeping on her own, in pencil, for nobody in particular: eleven children who had not borrowed a single book in the whole of last year, and who now came in every week. One of them, a quiet Year Seven boy, had read every book in a dragon series and asked if there were more. There were not. Mrs. Adeyemi had ordered him the next author she thought he might like.

"That's real," she said. "That happened because of the wall. I don't want to lose that."

On Friday afternoon Boaz found Tara and the others at the Whetstone. He had been thinking.

"I worked out what bothered me," he said, sitting down. "It's the question nobody asked. Every metric can be gamed. So before you fix the loopholes, you have to ask: what are we really protecting?"

"The reading," said Opal.

"Whose reading, though?" said Boaz. "Because the wall's protecting some of it and hurting some of it. Those eleven kids Mrs. Adeyemi found, the wall is the best thing that ever happened to them. But there's a girl in my form who used to read enormous fantasy books, seven hundred pages, and she's stopped. She says long books are bad for the class total, because you log the same minutes and you don't get the feeling of finishing. She's reading short ones now so she can say she's done something." He spread his hands. "So who loses if we only chase the number? She does. And the next year's kids, who'll learn that reading is the thing you do to move a cardboard climber."

"So scrap the wall," said Loop.

"Then the eleven kids lose," said Boaz. "I don't know the answer. I just think you fix the goal, not just the loophole. And nobody's even said what the goal is out loud."

"Then reward books instead of minutes," said Loop. "Reward books and you get books. That's how the loop works."

"Then the stand-in is books, and everyone reads the thinnest book in the library," said Opal. "They already are. Say what we wanted in words first."

"Reward pages, then," said Loop. "The loop feeds whatever it measures."

"Then that metric gets gamed too," said Boaz. "Picture books with big print. Skimming. Lying. Every metric can be gamed."

"Count how many kids come back to the library on their own, without a prize," said Opal slowly. "That's closer to the thing."

"That's closer," Boaz agreed. "And the second you put a prize on that, someone will walk in, touch a shelf and walk out again."

They all laughed, but not very much. It was the kind of laugh you give a problem you can see the shape of and cannot put down.

Opal looked at her napkin with its two overlapping circles. Danya had coloured the overlap in.

On Monday Dr. Hale called a short meeting in the library. She had to write a report for the governors about the wall, and she wanted to hear from the students who helped there.

She was not a villain either. Tara had known her for four years. Dr. Hale had taken over a school with falling numbers and a leaking roof and had fixed the roof first, because, she said, nobody learns anything with a bucket on their desk. She had to show the governors that things were working. The governors had to show the council. The council had to show someone else. That was how money got to schools. A number on a wall was something you could put in a report. "Children feel slightly more at home in the library" was not.

"I've heard about the audiobooks," said Dr. Hale. "And the bus. Mr. Whitlow wants stricter logging. Mrs. Adeyemi wants to keep it because of the children she's found. Some of you think it's made reading worse." She looked round at them: Mrs. Adeyemi on her step-stool, Opal with her notebook, Loop with his phone face down for once, Boaz by the window, Danya by the returns trolley pretending to sort books. "I'm being asked whether we roll it out to maths and science next year."

Then she looked at the shelves, where Tara was standing with an armful of returns.

"Tara," said Dr. Hale. "You've been in here every Wednesday for two years. You've watched what happened better than anyone. You haven't said anything. You never do." She smiled a little. "Should the wall stay?"

The library went quiet. The old clock above the door ticked. Somewhere a page turned, and nobody was logging it.

Tara thought of the eleven names in pencil. The audiobook running all night in an empty room. The girl with her seven-hundred-page books on a shelf at home. Mrs. Adeyemi's face on that first Friday, glowing. Dr. Hale's leaking roof. Two circles on a napkin, overlapping.

Everyone in the room was partly right.

Tara opened her mouth.

 

 

(Write the sentence Tara says.)

 

The spark dimmed to a warm dot and settled back on the anvil. The Green was just the Green again.

Back on the Green

At the end of term Danya's class started a sticker chart for "kind acts". Within a week, people were doing kind acts mostly where the teacher could see them, and arguing about whether holding a door counted twice if you held it for two people.

Danya came home and said, "The sticker chart's eating the kindness."

Tara laughed so hard she had to put her tea down.

The next week Danya asked his teacher if the class could stop counting the kind acts and just talk about one, on Fridays, without stickers. The teacher said she would think about it.

Afterword. The idea in this case has a name: Goodhart's law. It is often put like this: when a measure becomes a target, it stops being a good measure. A number that usually travels with something you care about will drift away from it once people are rewarded for the number itself, often without anyone cheating at all. Schools, hospitals, companies and apps all run into it. You can play with what a feedback loop rewards in SignalForge, with turning a fuzzy goal into a measure in InquiryForge, and with protecting a shared thing in CommonsForge.