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

Everybody in town called it the Seat. It was a broad, flat ledge of grey rock where the river bent below the old mill, smooth as a kitchen table and warm by eleven in the morning. Below it the water went from ankle-deep to deep enough to jump into in about three steps, and the jump was the first brave thing almost every child in town had ever done. Upstream, where the river slowed under the willows, the anglers stood in their waders from dawn, and had done since before anybody's grandparents could remember.

Tara had swum at the Seat every summer of her life. Her mother had swum there. There was a photograph on the fridge at home of her grandmother, aged about twelve, mid-jump, with her knees tucked up and her mouth wide open. It was the only free cool place in town. In July, when the flats over the shops were like ovens, half the town ended up there by four o'clock. It was, Tara thought, the one place in town that belonged to everybody at once.

That was the trouble.

The summer before, the bank above the Seat had begun to crumble. So many feet went up and down it that the grass had worn away and the soil had started to slide into the water in brown fans after every rain. There was litter: cans wedged in the roots, crisp packets in the reeds, once a whole broken deckchair. And the anglers said the trout were going. Old Mr. Pell, who had fished there for sixty years, said he caught in a whole morning now what he used to catch before breakfast.

By September the council had had enough complaints from enough people to call a meeting. It was held at the Whetstone, because the town hall's heating was broken, and Tara was at the back table with her history coursework, as usual. She had not meant to listen. She listened anyway.

Councillor Dunmore was a brisk woman in a green coat who had clearly had a long day. She did not waste time.

"We have three problems," she said. "Erosion. Litter. Fish stocks. And one more: our insurers have written to say that if anyone is injured on that bank in its present state, the council could be liable. So we have a plan." She held up a drawing. "A fence along the top of the bank. One gate. Season permits for swimming and fishing, twenty pounds each, which pays for a warden and the repairs. No permit, no entry. It's simple, it's fair, and it pays for itself."

There was a murmur. Some of it was relieved. Mr. Pell nodded slowly; a fence would keep the crowds away from his pools. Some of the parents nodded too: a warden sounded safe.

But a lot of the murmur was not relieved at all. Twenty pounds was nothing to some families and a great deal to others. Tara thought of the boys from the flats over the launderette, who were at the Seat every afternoon in July because there was nowhere else to go.

"Shall we take a sense of the room?" said the councillor.

"Can I ask something first?"

It was Ada, who was in Tara's year. Ada designed games: board games, card games, the house rules her four younger brothers and sisters lived by. She was famous for saying that every argument in her house was really an argument about a bad rule.

"Of course," said Councillor Dunmore.

"Rules aren't wishes, they're incentives," said Ada. "So I always ask: how will people actually behave? If keeping the rule costs more than breaking it, they'll break it. A fence along a river is about a mile long. The boys from the flats will be over it in a week, and then they'll be swimming somewhere with no warden at all." She paused. "And who made the rule? Was it the people who use the river?"

"The council made it," said Councillor Dunmore, a little stiffly. "On behalf of the people."

"Did anyone ask the swimmers and the anglers?" said Ada. "Together, I mean. In the same room."

The councillor opened her mouth, and then closed it. Nobody had.

Casimir was a quiet, careful boy who kept the fishing club's catch book. He was the youngest member by forty years and had inherited the job because he was the only one whose handwriting anybody could read. He had brought the book with him: a fat green ledger, every page ruled into columns. Date. Angler. Fish caught. Fish kept. Fish returned.

He stood up and opened it on the table nearest the front.

"Can I show you something?" said Casimir. "It's about the trout. Take what it grows back, and no more. That's the whole thing, really."

He found a page near the back.

"The river warden's survey says there are about a thousand trout in our stretch," he said. "A river like ours regrows about a fifth of that every year. New fish hatch and grow big enough. So roughly two hundred new trout a year. That's the refill rate. A shared thing has a refill rate."

He turned the page.

"Last year, between the club and everyone else, the town took about two hundred and sixty. That's sixty more than the river makes. We're taking more than it makes, every year. And it's not about this summer. It's about next. If we keep doing it, after five years there'd be about seven hundred. Then fewer, faster, because there'd be fewer fish to make the new ones."

Mr. Pell grunted. "That's what I've been saying."

"I know," said Casimir. "But it's not only the swimmers, Mr. Pell. Some of those sixty are ours. The club takes more than it used to as well."

Mr. Pell looked as if he might say something sharp. Then he looked at the ledger, at his own name in Casimir's careful handwriting, line after line, and he said nothing.

Danya, who had come with Tara and was sitting on the floor by the radiator, put his hand up.

"How do we know about the fifth?" he said. "The regrowing bit."

"Good question," said Casimir, and he meant it. "The warden's survey. And it's not exact. Some years it's more, some years less. But it's the best number we've got, and every year we take more than even a good year makes."

"So the fence protects the fish," said Councillor Dunmore. "Fewer people, fewer fish taken."

"Maybe," said a girl by the window. "Or maybe it just moves the problem downstream."

It was Oriel. She volunteered with the river warden on Saturday mornings, walking the bank with a litter picker and a clipboard, and she had the slightly weathered look of someone who spent a lot of time outdoors in bad weather on purpose.

"There's been a 'no litter, fine fifty pounds' sign at the Seat for nine years," said Oriel. "I've counted. In nine years, nobody has ever been fined. Not once. A fine nobody checks is just a sign. Everyone knows nobody's watching." She shrugged. "People keep a rule when they know it'll be noticed. Not when it's scary. When it's noticed."

"Which is why a warden," said the councillor. "Paid for by the permits."

"One warden," said Oriel. "For a mile of river, all summer, from six in the morning until dark. Who's going to see it? He can't be everywhere." She looked round the room. "But the swimmers are there every afternoon. The anglers are there every morning. Between them, there's somebody on that bank almost every hour of daylight. They see everything. They just don't think it's their job to say anything."

"So make it their job," said Ada slowly.

"Let the people who use it do the watching," said Oriel. "A notebook on a post at the top of the path. Who came, what they took, anything that's wrong. A rota. A neighbour saying 'oi, pick that up' carries more weight than a sign. A notebook on a post beats a fence."

"That sounds very nice," said Councillor Dunmore. "It also sounds like something that falls apart in about three weeks."

"It might," Oriel agreed. "Things do. But the fence falls apart in one week, when someone cuts a hole in it."

The meeting ended without a vote, which annoyed the councillor and relieved almost everyone else. She gave them a month. "Bring me a plan that deals with all four problems," she said, "or the fence goes up in the spring."

Ada did something that, as far as anybody could remember, nobody in the town had done before. She booked the back room of the Whetstone for a Sunday afternoon and invited the anglers and the swimmers. Both. In the same room.

It started badly. Mr. Pell said the swimmers churned up the water and frightened the fish. A mother from the flats said the anglers acted as if they owned the river. A boy said the anglers left fishing line in the reeds, which got wrapped round swimmers' legs. An angler said the swimmers left everything else in the reeds. For twenty minutes it was mostly people telling each other what was wrong with them.

Ada let it run. Then she stood up with a marker pen and a big sheet of paper.

"Design for how people actually behave," she said. "Not how we wish they'd behave. So: when do the swimmers come?"

"Afternoons," said the mother. "After school. Weekends."

"When do the anglers fish?"

"Dawn," said Mr. Pell. "The fish feed early. By ten, it's not worth it."

Ada drew two blocks on the paper, morning and afternoon, and they did not overlap.

"So you hardly use the river at the same time," she said. "You've been fighting about a place you take turns in already."

There was a pause.

After that it went faster. The anglers agreed a catch limit: the club would keep no more than the river made, about two hundred a year between them, and Casimir would keep the book. The swimmers agreed a litter rota: one family a day would do a sweep before they left. The anglers would collect their own line. Everyone agreed to keep the mornings quiet until ten, for the fish, which cost the swimmers nothing because they were never there anyway. And the older boys from the flats, who knew every inch of the bank, offered to rebuild the worn path with stones from the old mill wall, so that people would stop climbing down the crumbling bit.

"Who made these rules?" said Ada at the end, holding up the paper.

"We did," said Mr. Pell, slightly surprised to hear himself say it.

"Then you'll probably keep them," said Ada. "That's how it usually works."

Oriel nailed a notebook in a plastic sleeve to a post at the top of the path. On the first page, in her neat handwriting, she wrote: The Seat. Who came. What we took. What needs fixing.

A month later, the meeting was held again at the Whetstone. The heating in the town hall was still broken.

Councillor Dunmore had both plans on the table in front of her: the fence, drawn by the council's surveyor, neat and expensive, with a cost and a timetable; and the users' plan, written in marker pen on a big sheet of paper, with a notebook full of a month's entries beside it. The notebook said the litter rota had held for four weeks. It said the club had kept eleven trout and returned forty. It said the new path had been half-built by two boys from the flats and a retired engineer who had turned up with a wheelbarrow. It also said, on one page, in angry capitals, that somebody had left a barbecue tray burning on the rock, and nobody knew who.

The councillor was not a villain either. She had a budget that had been cut three years running. She had an insurance letter that frightened her. She had a real duty of care: if a child slipped on that bank and was hurt, it would be her name in the paper, and her conscience at night. A fence was something she could point to. A notebook on a post was not.

She looked round the room at all of them. Casimir with his ledger. Oriel with her clipboard. Ada with her marker-pen plan. Mr. Pell, who had come in his waders. The mother from the flats. Danya, cross-legged by the radiator.

Then her eyes stopped at the back table.

"You," she said. "Tara, isn't it? Your grandmother's in the library's local-history display, mid-jump off the Seat. You've swum there every summer. And you're the only person in this room who hasn't said a word at either meeting." She folded her hands. "So. The fence, or their plan. What do you think?"

The café went quiet. Outside, a bus went past in the rain. The coffee machine sighed.

Tara had listened to all of them. Mr. Pell's sixty years. The boys from the flats, with nowhere else to go. Casimir's sixty fish too many. Oriel's nine years of a sign nobody checked. Ada's two blocks on a sheet of paper. The barbecue tray, burning. The councillor, lying awake.

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

In the spring the Green's community fridge, the one outside the kitchen where people left food for anyone to take, kept ending up empty by eight in the morning, and then full of things nobody wanted.

Danya stood looking at it for a long time with his hands in his pockets. Then he went and found a notebook and a pen on a string, and taped them to the side of the fridge.

On the first page he wrote: Who left something. Who took something. What we need.

Afterword. The idea in this case is the commons: something shared, like a river, a field or a fishing ground, that anyone can use and that can be used up. For a long time people thought the only choices were to sell it off or let a government police it. The political scientist Elinor Ostrom, who won the Nobel Prize in economics in 2009, studied real places all over the world where the people who used a shared resource had kept it going for generations. They usually did it by making the rules together, watching each other, taking no more than it renewed, and settling arguments cheaply and close to home. It does not always work. But it works more often than people expected. You can play with a shared resource and its refill rate in CommonsForge.