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

It was the third week of July, and Tara was practising being somebody who lived on her own.

It had been her mum's idea. In September Tara would be in a flat, two hundred miles away, with five people she had not met yet and a kitchen she had only seen in a photograph. She had saved the deposit. She had a plan for the rent. But she had never in her life done a week's shopping by herself, for herself, with her own money.

"Do one now," her mum had said. "While you can still ring me from the aisle. Thirty pounds. A week of food for one person. Breakfast, lunch and tea. Go on."

So here she was, on a Saturday afternoon, in the big supermarket by the ring road, with a basket, a list on the back of an envelope, and thirty pounds in her purse in actual notes so that she could feel it go.

Her mum had given her one piece of advice, the same one she had been giving for as long as Tara could remember. She had said it in kitchens and car parks and at the checkout, every week for seventeen years.

Always buy the big one. It works out cheaper.

Tara started in the cereal aisle. There were two boxes of the one she liked, side by side. One was the ordinary box. The other was enormous, nearly twice as tall, with a bright yellow star on the front that said BIG VALUE!

BIG VALUE — 750g — £3.75.

500g — £2.25.

She reached for the big one without even thinking. It was what you did. It was what her family had always done. It felt grown-up, and careful, and sensible, and like being her mum.

Then she stopped, with her hand on the box, because she had just seen something small and grey on the shelf tag underneath it, and it made her want to ring someone.

"I was going to buy the biggest of everything," Tara said.

She had rung the back table at the Whetstone café, on speaker. Danya answered. He was there, he said, eating a scone and doing a crossword with Redline, who marked up everything she signed with a red pen, including permission slips for school trips, and who was never alarmed by anything.

"I was going to say it like this," said Tara, holding the phone in one hand and the cereal box in the other. "The big pack is always cheaper. Mum's always said so. It's the one rule I know about shopping. So I'll just fill the basket with the big ones, and the thirty pounds will go further. Simple."

She already had the big box half in the basket.

It felt good. It felt like something she did not have to think about.

"Read the small print out loud," said Redline calmly. "Before you put it in."

"It's cereal," said Tara. "There isn't any small print."

"There's always small print," said Redline. "Look at the shelf. Under the price. What does it say?"

Tara crouched down. The shelf tag under the big box had two prices on it. A big one, £3.75. And underneath, very small, as if it did not want to be noticed: 50.0p per 100g.

She looked at the tag under the ordinary box. £2.25. And underneath, just as small: 45.0p per 100g.

She read them both out loud, slowly.

"That one's ordinary," said Redline. "That one isn't. Which costs more per spoonful?"

"Says who the big one isn't cheaper?" said Danya, through scone. "It's got a star on it. It says BIG VALUE."

"The star says it," said Redline. "The tag says something else. Which do you believe?"

Tara looked at the yellow star, and then at the little grey numbers.

"How do we know which one's really cheaper?" Danya asked.

Nobody knew. That was the problem.

"I've got it," said Tara, still crouching in the aisle. "It says it. The little number. Fifty pence for every hundred grams in the big one. Forty-five in the small one."

"So the small one's cheaper," said Danya. "Per bit."

"Per hundred grams," said Redline. "Do the sum yourself, so you know the tag isn't lying either. The big box is three pounds seventy-five for seven hundred and fifty grams."

Tara did it on her phone's calculator. Three hundred and seventy-five pence, divided by seven and a half hundreds. Fifty pence per hundred grams.

"And the small one," said Redline. "Two pounds twenty-five for five hundred grams."

Two hundred and twenty-five, divided by five. Forty-five pence.

"So if I buy the big one," said Tara slowly, "I pay more for every bowl. Five pence more for every hundred grams. For the one with BIG VALUE written on it."

"Never sign anything unread," said Redline. "Even a cereal box. The star isn't a promise. It's a star."

A man in a green apron was stacking the shelf beside her, pulling boxes forward from the back so they lined up at the front. His badge said Mr Ahmed — here to help.

"You're right, you know," he said, without stopping. "That one catches people out. It used to be cheaper per gram, the big one. Then they changed the price on it in the spring and nobody noticed. Most of the time the big ones are cheaper. Most of the time. Not always."

"Then how do you know?" said Tara.

"I read the little number," said Mr Ahmed. "Every time. I've worked here eleven years."

Widen came on the call next, because Danya passed the phone across the table. Widen kept a notebook of what things cost, from bus fares to bridges, and could not hear a number without asking what it should be compared with.

"Compared with what?" said Widen. "You've done one aisle. Don't make a new rule from one box. That's as bad as the old rule. Do the next one."

Tara went to the pasta. There were two bags of the same spaghetti.

1kg — 90p. 9.0p per 100g.

500g — 55p. 11.0p per 100g.

"Here the big one wins," said Tara, surprised. "Nine pence against eleven. The big bag's cheaper per hundred grams."

"So Mum's rule is right here," said Danya.

"And wrong at the cereal," said Widen. "Same rule. Different answer. One number on its own tells you almost nothing. The pack price tells you what you hand over at the till. The little number tells you what each bit costs. Is that a lot, or does it just sound like a lot? You can only tell by the little number."

Tara went along the aisles with the phone held in front of her like a torch, reading little numbers out loud. Rice: big bag cheaper. Teabags: the box of eighty was dearer per bag than the box of forty, which was on offer. Tins of tomatoes: seventy-five pence each, or 3 for £2, which worked out at about sixty-seven pence each.

"Mum's rule works about half the time," said Tara.

"So it's a coin toss," said Danya.

"It's a coin toss you don't have to make," said Widen. "Because they've printed the answer on the shelf. Every time."

Mr Ahmed had followed her round with his trolley, in a slow, friendly way, as if this were the most interesting thing that had happened all shift. At the biscuits he stopped and held up a packet of the chocolate ones Tara's family always bought.

"Here's one for your list," he said. "These were three hundred grams for a pound last year. Still a pound now. Same packet, same picture. Look at the weight."

Tara looked. 250g.

"They took fifty grams out and kept the price," said Mr Ahmed. "Nobody noticed, because the packet looks the same, and the price on the front is the same. The only place it shows is the little number on the shelf." He put the packet back, neatly. "It went up from about thirty-three pence per hundred grams to forty. That's a price rise. It just doesn't look like one."

"Can they do that?" said Danya, on the phone.

"They can," said Mr Ahmed. "They say it's that or put the price up. It happens to all sorts. Crisps. Chocolate. Washing-up liquid. That's why I always tell people the same thing. Ignore the star on the box. Ignore the size of the box. Read the little number."

"Widen the view," said Widen. "Last year's packet is a comparison too."

Tara wrote biscuits — check the weight on her envelope, in very small letters, under everything else.

Marcus was at the café too, with an envelope labelled SUMMER, and he took the phone last. Marcus kept envelopes for everything and gave every pound a job.

"Where are you now?" said Marcus.

"Salad," said Tara. She was in front of the bags of salad leaves. "Big bag, two hundred and fifty grams, one pound fifty. That's sixty pence per hundred grams. Small bag, a hundred and twenty grams, ninety pence. That's seventy-five. So the big bag's cheaper. Easy one."

"How much do you actually need?" said Marcus.

"What?"

"How much salad do you eat?" said Marcus. "Living on your own. In a week. Before it goes slimy at the bottom of the fridge."

Tara thought about it honestly. At home, salad was something that happened in a big bowl on a Sunday. On her own, it would be a handful in a sandwich, a few times.

"About the small bag," she said. "Maybe a hundred and twenty grams. The rest would go in the bin."

"Then do the sum for what you'll eat," said Marcus. "Not what you'll buy. If you buy the big bag, you pay one pound fifty. And you eat a hundred and twenty grams. So what you've paid for what you've eaten is one pound fifty for a hundred and twenty grams."

Tara did it on the calculator. One hundred and fifty pence, divided by one point two. A hundred and twenty-five pence per hundred grams.

"One pound twenty-five," she said. "For every hundred grams I actually eat. Against seventy-five for the small bag, which I'd finish."

"Give every pound a job," said Marcus. "The pound you spend on salad you throw away hasn't got one. It's the cheapest bag in the shop, and the dearest salad you'll eat all week."

Tara put the big bag back.

"So it's not even the little number," she said slowly. "It's the little number, and then how much of it I'll use."

"Write the plan before the money comes," said Marcus. "That's the plan."

"Let me listen first," Tara said, to herself, in the middle of the salad aisle.

She took the envelope with her list on it and turned it over. On the back she wrote three headings: the pack price, the little number, what I'll actually use.

Under the first she wrote cereal £3.75 or £2.25, pasta 90p or 55p, salad £1.50 or 90p. Under the second, 50 v 45, 9 v 11, 60 v 75. Under the third she wrote all the cereal, all the pasta, only 120g of salad.

Then her phone buzzed. It was Juno, who had seen her post a photo of the cereal aisle with HELP written on it.

Just get whatever's on offer!! Juno had written. 3 for £2, buy one get one free, all that. An offer's an offer. My mum gets everything on offer and she says it saves her loads.

Juno was not wrong either. The tins were cheaper three for two. Tara had done the sum.

Then her mum rang.

"How's it going?" said her mum. "Remember: always the big one."

"Mum," said Tara. "The big cereal's dearer per hundred grams than the small one."

There was a pause.

"Is it?" said her mum. "Well. I fed four of us on buying big for twenty years, you know. It mostly worked. With four of you eating, nothing ever went off." She laughed. "You lot ate the big bags of salad in about ten minutes."

That was true too. In a house of four, the big bag of salad was gone by Monday. In a flat of one, it would be gone by Thursday, into the bin.

Mr Ahmed went past with his trolley of boxes. "Big ones are usually cheaper per gram, love," he said cheerfully. "Usually. Read the little number."

"I think we missed someone," Tara said, mostly to herself. "Me. In September I'll be one person. Mum's rule was a rule for four people. It worked because there were four of us. Nobody ever said that part out loud."

In her basket were the small box of cereal, the big bag of pasta, the small bag of salad and three tins of tomatoes. In her purse were thirty pounds, still uncounted. On her phone were Juno's an offer's an offer, and her mum's always the big one. In her head were Redline's read the small print out loud, Widen's compared with what, and Marcus's how much do you actually need.

And in her hand was the envelope, with her list on one side, and a blank space at the very top, above everything, where a rule would go.

She took the pencil out of her bag.

 

 

(Write the rule Tara writes at the top of her first shopping list.)

 

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

Back on the Green

That afternoon Lan, who was seven, was choosing between a big bag of sweets and a small one at the corner shop. The big bag was more money but had a lot more sweets.

Danya asked her how many sweets she could eat before tea.

Lan said four, because her mum had said four.

Danya asked what would happen to the rest of the big bag.

Lan said her brother would eat them.

She bought the small bag. She ate four sweets before tea, and kept the other two for the next day, in her pocket with the zip, where her brother could not find them.

Afterword. The idea in this case is unit price: the price per kilogram, per litre or per hundred grams, which is usually printed in small type on the shelf tag. It lets you compare packs of different sizes fairly. Big packs are often cheaper per gram, but not always; prices change, and a pack can quietly shrink while its price stays the same. The second idea is what you will actually use. A big pack that is cheaper per gram can be the most expensive choice if half of it goes in the bin. Before you choose, read the small number, compare like with like, and price what you will eat, not what you will buy. You can practise comparing prices of different sizes in LedgerQuest, and reading the small print on everything in ClauseDojo.