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

The ring had three keys now. Brass, copper and silver. Inkling counted them twice, to be sure.

The spark rolled down the lane behind the print shop, and the lane became a square of shops. These shops sold scales, rulers, jugs with lines on them and magnifying glasses. A carved board over an arch said ASSAY COURT.

"What's an assay?" asked Beto.

"A test," said Danya. "Of what something is made of."

There was a little museum on one side of the court. A collector stood at its door with a wooden box.

"I want to give the museum my coin," she said. "I was told it is solid silver."

She opened the box. The coin was large and bright.

"The court would like it tested before it goes on show," said the museum keeper. "Would the company do it?"

Inkling leaned close. "I can name it," she said. "It's silver. Look how it shines."

"What exactly is being claimed?" said Claim.

A magpie sat on the edge of the box with a small notebook. Magpies like shiny things. But this one did not seem impressed.

"Solid silver, or silver on the outside?" said Claim. "Shine only tells you about the outside."

Inkling stopped. "Oh," she said. "So shine can't settle it."

"Then write the claim down first," said Claim. "One sentence." She held out her notebook and the pencil.

Inkling thought about it. Then she wrote: THIS COIN IS SILVER ALL THE WAY THROUGH.

A badger came over with a set of scales and a jug of water.

"What would change my mind?" said Prove. "Pick the test before the result."

"Why before?" asked Beto.

"Because if you choose the line after, you can always move it," said Prove. "A test that can't fail tells you nothing."

Prove gave them one fact and no more. "Every material has its own weight for its size. Solid silver weighs about ten and a half grams for every cubic centimetre. So: what result would change your minds?"

Beto thought. "If it came out a lot less than ten and a half," he said, "it couldn't be solid. Something lighter would be inside."

"How much less?" asked Prove.

"Under ten," said Beto. "Under ten, and it isn't solid silver."

Danya wrote the line in the notebook, under the claim.

Try the Packet now, if you have it. Then read on.

A giraffe ducked under the arch with a folding ruler.

"Compared to what?" said Yardstick.

The museum had a coin it knew was real silver. Yardstick weighed it on the museum's own scales. One brass pan had been dented and beaten flat again, and under the rim was a tiny scratched hammer.

"Who mended your scales?" asked Inkling.

"Before my time," said the museum keeper. "Children, I was told. The ones who first tested things on this court. They left a mark, but never a name."

Inkling had seen that hammer on a sign, a ledger and a door. Somebody had been here before them, on every street.

The scales said 31.5 grams.

Then she lowered it into the jug. The water rose by 3 cubic centimetres.

Beto did the sum in the notebook. "Thirty-one and a half grams for three cubic centimetres," he said. "That's ten and a half for each one. Just what real silver should be."

"So the method works," said Beto.

"A number needs a neighbour," said Yardstick.

Danya tested the collector's coin in exactly the same way.

The scales said 27 grams. The water rose by 3 cubic centimetres, the same as the known coin.

"Same size, but lighter," said Danya. "Twenty-seven for three is nine."

Nine was under the line. Everyone looked at the notebook. The line had been written before the test, so nobody could argue with it.

"How do we know what it is?" asked Danya. "We know it isn't silver all the way through. It's silver on the outside, and something lighter inside."

The collector was quiet for a moment. Then she smiled.

"I'm glad to know," she said. "I would rather the museum told the truth."

The museum keeper wrote a new label: SILVER-PLATED COIN. TESTED ON ASSAY COURT. The collector gave it anyway.

The court gave the company an iron key. Beto put it on the ring. Four keys, from four streets.

The spark rolled back up the lane, and the lane was just the lane behind the print shop.

Back on the Green

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

Inkling's friend said her trainers made her run faster. Inkling asked, "What would change your mind?" They raced twice, once in each pair of shoes.