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

It was nearly dinner time, and the Green was going gold in the low sun. Lan was hanging upside down from the climbing frame, and Inkling was lying in the grass counting clouds, when the spark came skipping past them. It did not go to the forge cart, or the print shop, or the ferry. It went straight across the grass to Bà Tư's house and tapped on the kitchen window. Tap, tap. Tap, tap.

"It wants us in the kitchen," said Inkling.

"I'll go first!" said Lan, and dropped off the frame and ran.

Bà Tư's kitchen always smelled of something. Tonight it smelled of sugar and lemons, and every surface was covered with the beginnings of tomorrow. Tomorrow was the party. Bà Tư had been planning it for weeks. There would be a long table on the Green, and lanterns in the trees, and the whole street invited, and in the middle of the table there would be a jelly.

Not just any jelly. Bà Tư's jelly. It was famous. It was tall and clear and golden, and it had pieces of fruit hanging inside it like little suns, and when you put it down on the table it went wobble, wobble, wobble for a long time, and everybody laughed.

Bà Tư was standing at the counter, looking into a big glass bowl. She did not look happy.

"Watch closely, little one," she said to Lan, and tipped the bowl very gently.

The jelly sloshed. It sloshed like soup. Golden juice ran up one side of the bowl and down again, and the pieces of pineapple bobbed about inside it.

"It should be wobbly by now," said Bà Tư. "I made it this morning. It's had all day."

Lan went pale.

"Oh no," she said. "Oh no, oh no. Bà, I told everybody. I told Mrs. Ama, and the twins, and the man with the dog. I said, Bà's jelly is the best jelly in the world, and I promise it will wobble."

"You promised?" said Inkling.

"I promise," said Lan miserably. "I mean, I promised. I always promise. I just say it, and then it's out."

It was true. Lan always went first, and she always promised before she had counted. Usually it worked out. Tonight it had not.

Inkling walked all the way round the counter, looking at the jelly from every side. Then she stood up very straight.

"I know what it is," she said. "I can name it. It's the fridge!"

"Which fridge?" said Bà Tư.

"The garage fridge," said Inkling. "You put the jelly in the garage fridge this morning, because the kitchen one was full of party things. I saw you. That fridge is old. It hums. It's too warm. That's why the jelly didn't set." She was already picking up the bowl. "We just move it to the kitchen fridge, and by tomorrow it will be perfect."

"Come on, quick!" said Lan, holding the kitchen fridge door open.

"Wait," said somebody on the window sill.

A grey raccoon was sitting there, half in and half out of the open window. He had a black mask round his eyes, and a little smile, and he was holding a glass jar in each paw. In one jar a seedling grew tall. In the other, a seedling grew short.

"I'm Compare," said the raccoon. "I came in with the spark. May I ask a question?"

Inkling stopped, holding the bowl.

"Is the fridge the only thing that's different about this jelly?" said Compare.

Inkling opened her mouth. Then she closed it.

"No," said Bà Tư slowly. "No, it isn't. I usually measure the water with my blue jug. This morning the jug was full of flowers for the table, so I just poured from the kettle. And I usually put tinned peaches in. This time the market had beautiful fresh pineapple, so I used that instead."

"So that's three things," said Compare. He held up his two jars, side by side. "These two plants had the same pot, the same soil, the same water. Only the light was different. So when one grew tall, I knew it was the light." He put the jars down. "Your jelly has three differences. If you move it to the other fridge, and it still doesn't set, what have you learned?"

"That it isn't the fridge," said Inkling.

"And if you move it, and it does set," said Compare, "is that because of the fridge, or because it simply had one more night?"

Inkling put the bowl down very carefully. Her cheeks had gone hot.

"I named it too soon," she said. "Again."

"You named a suspect," said Compare kindly. "That's not bad. That's a start. But a guess said loudly is still a guess."

Nobody, it turned out, agreed about which suspect it was.

"It's the fridge," said Inkling, who still thought so, even if she wasn't allowed to say it loudly. "That fridge hums. A fridge that hums is a fridge that's trying too hard."

"It's the water," said Lan. "Bà always uses the blue jug. Always. If you don't use the blue jug, you put in too much, and then it's too wet to wobble. That's obvious."

Bà Tư folded her arms. "It is not the pineapple," she said. "That pineapple was beautiful. I chose it myself. It smelled like summer. Nothing that smells like that ever ruined anything."

They all looked at each other. Three people, three suspects, and every one of them sure.

"This is my favourite part," said Compare happily. "When everybody is sure, and everybody is different. It means at least two of you are wrong, and nobody knows which two."

"That's not very nice," said Lan.

"It's very nice," said Compare. "It means we get to find out."

"Change one thing at a time," said Compare. "Keep everything else the same."

He hopped down from the window sill and stood on the counter among the lemons.

"Three suspects," he said. "The garage fridge. The water poured without measuring. And the fresh pineapple. Let's make three little jellies, each with only ONE thing changed from the one that failed. Then whichever one sets will point at the culprit."

"Two jars, one difference," said Lan, looking at his jars.

"Three bowls, one difference each," said Compare. "Same bowl, same water, same fridge, except for the one thing you're testing."

Bà Tư thought about it, and then she nodded, and rolled up her sleeves. "You try," she said to the girls, "and I'll hold the pan."

She made a small pan of jelly exactly the way she had made it that morning: water poured from the kettle without measuring, the same packet of jelly crystals, and fresh pineapple. While it was still warm, Inkling and Lan poured it out into three small glass bowls.

Then they changed one thing in each.

Bowl one stayed just as it was, with its fresh pineapple and its unmeasured water. But instead of going to the garage fridge, it went into the kitchen fridge, on the top shelf. "If this one sets, it was the fridge," said Inkling.

Bowl two was made again with the water measured exactly in the blue jug, once the flowers had been taken out of it. It had fresh pineapple, like the first jelly, and it went into the garage fridge, like the first jelly. "If this one sets, it was the water," said Lan.

Bowl three had the unmeasured water, like the first jelly, and it went into the garage fridge, like the first jelly. But it had no pineapple at all. "If this one sets," said Inkling, and then she stopped, because she did not want to name it too soon. "Then we'll see," she said.

They stuck a paper label on each bowl: FRIDGE, WATER, FRUIT.

"Same bowls, same jelly crystals, same night," said Compare, counting them on his fingers. "One change each. Now we wait."

That night, Lan could not sleep.

She lay in bed and looked at the ceiling, and she thought about Mrs. Ama, and the twins, and the man with the dog. She thought about them sitting down at the long table on the Green, with the lanterns in the trees, and looking at the middle of the table, where the jelly should be. She thought about having to say, I'm sorry, it's soup.

"I said it too soon," she whispered to the ceiling. "I promised. And I didn't check. I never check."

She thought about Compare's two jars, and his calm little smile.

Then which change did it?

She turned over. She turned over again. Out of the window, across the Green, she could see the kitchen light still on in Bà Tư's house, and a small warm dot of spark sitting on the window sill, waiting.

"Lan," whispered a voice from the camp bed by the wall. "Are you awake?"

Inkling was sleeping over, because of the party. She was awake too.

"I can't stop thinking about it," said Lan. "I always promise. Before I count. Why do I always do that?"

Inkling was quiet for a while. "You promise things because you want everybody to be happy," she said at last. "Straight away. You can't wait for them to be happy later."

Lan thought about that. It was true, and it was strange to hear somebody else say it.

"You name things," said Lan. "Before you've looked properly. You named the fridge before Bà even finished tipping the bowl."

"I know," said Inkling, and sighed. "I see a thing, and the name jumps out of my mouth, and then it's too late."

"So I promise too soon," said Lan, "and you name too soon."

"It's funny," said Inkling. "I could see yours straight away. I couldn't see mine at all until Compare asked his question."

"Same," said Lan. "I could see yours. Not mine."

They lay in the dark and thought about that.

"Tomorrow," said Inkling, "if I start to name something too soon, you poke me."

"And if I start to promise," said Lan, "you poke me."

"Deal," said Inkling. "But not too hard."

At last she fell asleep.

In the morning they ran all the way across the wet grass.

Bà Tư was already in the kitchen with a cup of tea. She had not opened either fridge. "I waited for you," she said. "It's your test."

"Before we look," said Compare, who had appeared on the window sill again, "everybody guess. Which bowl will wobble?"

"FRIDGE," said Inkling at once. Then she made herself stop and think. "No. I don't know. I still think FRIDGE, but I don't know."

"WATER," said Lan. "I hope it's WATER. If it's WATER, it's just a jug, and that's easy."

Bà Tư sipped her tea. "Not FRUIT," she said firmly. "Anything but FRUIT."

"Good," said Compare. "Now you've each said what you think. In a minute, the jelly will tell you what's true. Those aren't always the same, and that's fine."

Inkling opened the kitchen fridge and took out bowl one, FRIDGE. She tipped it.

It sloshed.

"So it isn't the fridge," said Inkling quietly.

Lan ran to the garage and came back with the other two bowls. She put bowl two, WATER, on the table and tipped it.

It sloshed too.

"Not the water," said Lan.

Then, very slowly, she tipped bowl three, FRUIT, the one with no pineapple.

It wobbled.

It went wobble, wobble, wobble, all the way round, golden and clear, and stopped.

Lan gave a shriek and clapped her hands over her mouth. Inkling stared.

"Look at the pieces first," said a soft voice from the top of the cupboard.

A brown owl was sitting up there, among the cake tins. She wore round yellow glasses, and she was peering down at the table with her head on one side. Her name was Patternby.

"May I come down?" said Patternby. "I love a clue."

She fluttered down beside the big runny jelly from yesterday, the one that had started it all. Bà Tư had left it out on the counter overnight. Patternby put her face very close to it and looked.

"Hm," said Patternby. "Let me look more closely."

The big jelly was not quite all soup any more. In some places, out in the middle and at the edges, it had gone a little thick and gloopy. But right around each chunk of pineapple, it was thinnest of all. Each piece of fruit sat in its own little pool of runny juice, as if the jelly had melted away from it.

"What do the runny parts have in common?" said Patternby.

"They're all next to pineapple," said Inkling.

"A clue in every piece," said Patternby, pleased. "The pieces tell the rule. And your three bowls told it too. Only the bowl without pineapple set. The fridge didn't matter. The water didn't matter. The fruit did."

"Then pineapple is bad for jelly!" said Inkling. "No pineapple ever again. That's the answer."

She said it loudly and quickly, and then she heard herself saying it loudly and quickly, and she wished she hadn't.

There was a scuttling sound behind the fruit bowl, and a round orange crab came out sideways, holding up one claw like a pair of scissors. She had a long paper chain looped over her other claw, red and yellow and blue.

"Snip!" said the crab. "Only after yellow."

She held up the chain and snipped it. The cut came right after a yellow link. She moved along and snipped again. Right after a yellow link. And again. Red, blue, yellow, snip. Red, blue, yellow, snip.

"I'm Clippa," said the crab. "I cut at one pattern, every time. Snip, snip, the same place again. Not there. Only there."

The chain fell into little pieces on the counter, and every piece ended in yellow.

"Now me," said Patternby, hopping closer. "Make another chain, and cut it your way, and don't tell me your rule. I'll work it out from the pieces."

So Clippa turned her back, and Lan made a new chain from the paper box, red and blue and yellow and green, all mixed up. Clippa snipped it, quick as anything, snip, snip, snip, and swept the pieces into a pile.

Patternby looked at the pile. She turned every piece over with the tip of her wing. Some pieces were long and some were short. Some had four links and some had only one.

"What do the cut ends have in common?" said Patternby, mostly to herself. "Hm. This one ends in green. And this one. And this one. Every single piece ends in green."

"Snip!" said Clippa, delighted. "Only after green! You found my pattern."

"The pieces tell the rule," said Patternby. "They always do, if you look at all of them and not just one."

Inkling watched the snips. Then she looked at the runny jelly, and the little pools around the pineapple.

"Maybe," she said slowly, and this time she said it quietly, "maybe the pineapple cuts the jelly. Like Clippa cuts the chain. Maybe something in the pineapple snips whatever holds the jelly together."

"Find my pattern and you'll find me," said Clippa, and clicked her claw happily.

"Then is it all pineapple?" said Lan. "Every kind? Or only some?"

They all turned to look at Compare, who was sitting on the window sill again with his jars.

"Then which change did it?" said Compare, and smiled. "Try it and see. Change one thing at a time. Fresh pineapple. Pineapple from a tin. And fresh pineapple that's been cooked first. Same jelly, same water, same fridge."

"Why tinned?" said Lan.

"Because tinned fruit has been heated," said Bà Tư. "They cook it a little, to seal it in the tin." She took a tin of pineapple down from the cupboard, and then she took a few chunks of the fresh pineapple and put them in a small pan of water on the stove. "You watch from there," she said, pointing to a spot two steps back, "and I'll hold the pan."

The fresh pieces simmered for a few minutes, bubbling softly, and the kitchen filled with a hot, sweet smell. Bà Tư let them cool. Then the girls made three more little jellies, all exactly the same except for the fruit: FRESH, TIN, COOKED. Into the kitchen fridge they went, side by side on one shelf.

All morning they did other party jobs. They hung the lanterns. They counted the chairs, twice, because Lan wanted to be sure. They folded paper napkins into boats.

When the twins came past and asked whether the jelly would be ready, Lan started to say "I promise," and Inkling poked her. "We're testing it," said Lan instead. "We'll know at lunch."

And when Inkling looked at the three bowls through the fridge door and said, "It'll be COOKED, I know it will," Lan poked her back.

"Ow," said Inkling. "Fine. I don't know it. I think it."

"Thinking is allowed," said Lan. "Knowing comes at lunch."

At lunch they opened the fridge.

FRESH sloshed.

TIN wobbled.

COOKED wobbled.

"So pineapple isn't bad," said Inkling slowly. She was thinking hard. "Fresh pineapple is just wrong for this jelly. When it's been heated, it's fine. It's not a yes or a no. It depends."

"That's the best kind of answer," said Patternby, polishing her glasses on her feathers. "The kind you have to look closely to find."

There was just enough time.

Lan made the party jelly again, all by herself, with Bà Tư holding the pan. She measured the water in the blue jug, because she wanted to. She used tinned pineapple, because now she knew. And she poured it into the big glass bowl and carried it, very carefully, to the kitchen fridge.

At three o'clock, Mrs. Ama came across the Green with a plate of sesame biscuits.

"Lan!" she called. "Is the famous jelly ready? You promised!"

Lan opened her mouth to say yes, I promise, because that was what her mouth always did.

Inkling, standing beside her with a tray of cups, gave her a tiny poke in the elbow.

Then she closed it again.

"I don't know yet," said Lan. "I'll check first."

Mrs. Ama raised her eyebrows, and smiled, and went to put down her biscuits.

Lan waited. She went and helped with the chairs. She came back and opened the fridge and poked the jelly with one finger. Still soft. She waited some more. She came back. She poked it again. A little firmer.

At five o'clock, when the lanterns were lit and the long table was full, Lan opened the fridge one last time, lifted out the big glass bowl, and tipped it, very gently.

It went wobble, wobble, wobble, all the way round, golden and clear, with the pineapple hanging inside it like little suns.

Lan carried it out across the Green and set it down in the middle of the table, and it wobbled for a long time, and everybody laughed.

Mrs. Ama leaned over and poked it with her spoon, and it wobbled even more.

"You were very careful today," she said. "You didn't promise."

"I'd already promised once," said Lan. "And it went wrong." And then, because everybody was listening, she told them the whole thing: the soup jelly, and Inkling blaming the fridge, and Compare's three bowls, one change each, and the long night, and the bowl with no pineapple that wobbled, and Clippa's scissors, and the tin.

"So fresh pineapple is bad for jelly?" said one of the twins.

"No," said Lan and Inkling together. "It depends."

Inkling explained: fresh pineapple was wrong for this kind of jelly, but heated pineapple was fine, and you could only find that out by trying it one change at a time.

The man with the dog nodded slowly, as if he were going to try it at home.

"Ready," said Lan.

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

Back on the Green

The next week, Lan made a paper boat to sail on the pond, and it sank. She made another, and it sank too.

"It's the wind!" said Inkling at once. Then she laughed at herself. "Or it's the paper. Or it's how you folded it."

"Three suspects," said Lan. She made two boats exactly the same way, from the same paper, and then she changed just one thing: she folded one of them tighter. She put them on the pond side by side, in the same wind.

The tight one floated.

"Then which change did it?" said Lan, and grinned.

Inkling wrote it down in her notebook, in big letters, so she would not forget: ONE CHANGE AT A TIME.

Tell the story back to someone. Why did they make three bowls, and not just move the jelly to the other fridge?