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

Beto put an egg in an egg cup. It sat perfectly. He tried a grape. It rolled around. He tried a satsuma. It wobbled on top.

"Only an egg fits it," said Beto. "It's the shape."

"How do we know it's the shape and not luck?" asked Danya.

The spark slipped through the museum's back door into the workroom where the Sketcher and the Photographer work. They followed.

Margit was drawing on squared paper today.

"I've never seen it. I only read the letters," she said. "But this chain folds round a hollow. This part I'd bet on."

She inked the fold in firm lines. In the middle she left a round gap, like the hollow of an egg cup. It was eight squares deep.

"What goes in it?" asked Danya.

"The letters don't say," said Margit. "They only tell me about the chain. Anything that isn't part of the chain, I can't draw."

So she left the pocket empty. That was the honest thing to do.

Kemi came out of the cold room carrying her camera case. She laid the photograph on the table.

The fold was there, sharp and clear. So was the pocket. But inside the pocket sat a round grey smudge.

"Hang on," said Beto. He licked his thumb and reached for it. "Dirt on the picture."

"Wait," said Kemi. "My photograph shows what was there, not why. That smudge was really there. If you wipe it, you wipe out the best part."

Beto pulled his thumb back. He looked at the smudge more carefully. It was very round. It sat right in the middle of the pocket.

"Let's pin them side by side," said Kemi.

She laid a ruler across the smudge. Eight squares wide. Then across the pocket in Margit's drawing. Eight squares deep.

"It fits," said Beto. "No gap at all. Like the egg."

He tried to imagine other things in that pocket. A long thin one would stick out. A bigger round one would not go in at all. Only something exactly that size would sit there without a gap.

"So only one kind of thing fits that pocket," said Beto. "The shape settles it."

"Something small sits in that pocket," said Kemi. "Maybe a metal. Maybe a little sugar. It's not part of the chain, so it isn't in the letters."

"So my drawing was right," said Margit, "and it was missing something. Show me the photograph again. I want to be wrong if I'm wrong."

She studied it, smiling. "I'm not wrong. I'm just not finished."

Danya put the two pictures next to each other.

"How do we know who did what?" he asked. "Let's be fair to both."

Margit's drawing had the right shape: the fold and the pocket, eight deep. It came from the letters alone.

Kemi's photograph had the same pocket. And it had the partner sitting in it, which no letters could have shown.

"A prediction draws the chain," said Danya. "A photograph shows what the chain was holding."

"A blur tells you something too," said Kemi. "So does a smudge."

They pinned the fourth pair to the board. An empty pocket on the left. A filled pocket on the right.

The board's corner had broken off once and been glued back with a patch of card. On the card was a tiny hammer, drawn in pencil.

"Who used this board before you?" asked Danya.

"Children, I think," said Kemi. "Before this was a workroom, children pinned their pictures here. One of them mended the corner."

Beto had seen that hammer on a table leg, a mop handle and a jar lid. Somebody had been here before them every time, and had mended something, and had not left a name.

The spark slipped back out through the museum door, and the workroom was dark again.

Back on the Green

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

At home, Danya looked at a coin slot in the money box. He did not guess. He measured a coin and the slot with a ruler, then dropped the coin in.