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

Danya's coat zip had come apart at the bottom. He held one side in each hand.

"Each half is useless on its own," he said. "It only works when they meet."

"I could take it apart," said Beto. "Then we'd have two useless halves."

"Please don't," said Danya.

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

Margit was frowning at her drawing.

"I've never seen it. I only read the letters," she said. "This chain is a hundred and fifty links. Most of it I'd bet on."

She had inked a neat fold. But on one side there was a flat patch, drawn in pencil, with a question mark beside it.

"Pencil means I don't know yet," said Margit. "This patch is oily, and it's on the outside. That's odd. Oily parts usually end up inside."

Danya remembered the salad dressing. "Unless something covers it," he said.

Kemi came out of the cold room, stamping the cold out of her boots.

"I took it in the cold," she said.

The photograph did not show one chain. It showed two, side by side, exactly the same. Their flat patches were pressed together. Where the two chains met, there was a long groove running between them.

"Two of them," said Beto.

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

Beto picked up the scissors. "They're identical," he said. "If we cut the photo down the middle, we'd have one chain. That's all we need."

"Wait," said Kemi. "Look at the groove. It's only there when both of them are. Cut them apart, and the groove is gone."

Beto looked again. The groove ran right along the join. Neither chain had it alone.

"So the pair can do something one half can't," he said slowly. "Like the zip."

"My photograph shows what was there, not why," said Kemi. "But this is a pretty good why."

She showed them how she knew the two chains were the same. Every spiral matched. Every fold matched. Even the little bumps along the edge were in the same places, just turned round.

"Like your two hands," said Kemi. "The same shape, facing each other."

Danya held up his hands, palm to palm. They fitted. Then he held up one hand alone. It was just a hand.

Danya laid Margit's drawing next to the photograph. He turned it until the pencilled patch faced the second chain.

It fitted exactly. The odd flat patch was the place where the two chains touched.

"Your pencil marked the meeting place," said Danya. "You just didn't know there was anyone to meet."

"Show me the photograph," said Margit. She was smiling. "I want to be wrong if I'm wrong. I drew one chain, because I was only given one. The patch was the clue."

Danya did the sum on the back of his hand. Two chains of a hundred and fifty links. Three hundred links in the pair.

They pinned the sixth pair to the board. One chain on the left. A pair on the right.

The board was full now. Six pairs. In every one, the drawing and the photograph disagreed about something, and in every one, both of them had been useful.

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 the mending shed, Beto fixed the zip. He did not take it apart. He fed the two halves back together, one tooth at a time.