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

It skipped across the Green and tapped on Bà Tư's kitchen window. Tap, tap.

Inside, Bà Tư was looking into a big glass bowl. The party was tomorrow. The jelly was meant to be wobbly by now. It was not. It sloshed.

"Oh no," said Lan. "I told everybody. I said, I promise it will wobble!"

She had promised before she counted. She did that.

"It's the fridge! I can name it," said Inkling. "Bà Tư put it in the garage fridge. That one's old. It's too warm. Move it to the kitchen fridge and it will set."

She was already lifting the bowl.

A grey raccoon was sitting on the window sill, holding a jar in each paw. His name was Compare.

"Then which change did it?" said Compare. "Three things are different about this jelly. The fridge. The water, poured without measuring. And the fresh pineapple. If you just blame one, you will never know."

Inkling put the bowl down. Her cheeks went hot.

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

Bà Tư made a little more of the same jelly. They poured it into three small bowls.

Bowl one went in the kitchen fridge. Everything else was the same as the big jelly.

Bowl two had its water measured carefully. Everything else was the same.

Bowl three had no pineapple. Everything else was the same.

"Two jars, one difference," said Compare. "Three bowls, one difference each."

That night Lan could not sleep. She kept thinking of the guests, and the runny jelly, and her promise.

"I said it too soon," she whispered to the ceiling. "I didn't check."

In the morning they ran to the kitchen.

Bowl one, in the kitchen fridge: runny.

Bowl two, with measured water: runny.

Bowl three, with no pineapple: it wobbled!

A brown owl in round yellow glasses was peering into the big bowl. Her name was Patternby.

"Look at the pieces first," said Patternby. "Hm. A clue in every piece."

The big jelly was a little bit firm in some places. But right around each chunk of pineapple, it was runniest of all.

"The pieces tell the rule," said Patternby. "It's the pineapple."

"Then pineapple is bad for jelly!" said Inkling. "No pineapple ever again."

A round orange crab scuttled out from behind the fruit bowl, holding up one claw like a pair of scissors. Her name was Clippa.

She had a paper chain of red, yellow and blue links. She snipped it. Then again. Then again. Every cut came right after a yellow link.

"I cut at one pattern, every time," said Clippa. "Snip, snip, the same place again."

Inkling watched the snips. Then she looked at the runny jelly round the pineapple.

"Maybe the pineapple cuts the jelly," she said, more slowly this time. "Like Clippa cuts the chain."

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

"Change one thing at a time," said Compare. "Fresh, from a tin, and fresh but cooked. Same jelly, same fridge."

So they tried. Bà Tư opened a tin of pineapple, which had been heated when it was tinned. Then she simmered a few fresh pieces in a pan, while the children watched from a safe step back. Three more little bowls went in the fridge.

By lunch, the tinned bowl and the cooked bowl wobbled. The fresh one sloshed.

"So pineapple isn't bad," said Inkling slowly. "Fresh pineapple is wrong for this jelly. Cooked is fine."

Lan made the party jelly again, with tinned pineapple.

This time she did not promise. When the first guest asked, she said, "I don't know yet."

She waited. She poked it. She waited some more.

Then it wobbled, all the way round.

"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

Next week, Lan's paper boat kept sinking in the pond. Was it the paper, or how she folded it, or the wind? She made two boats the same, and changed just one thing.

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