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

Danya had taken a pen apart. The spring sat on his palm, a tiny spiral. He pulled it straight and let go. It curled up again.

"Why does it curl?" he asked. "How does it know?"

"It doesn't know," said Beto. "It's just made that way. I could take it apart more."

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

A long table stood by the window. One of its legs had cracked once and been splinted with a strip of wood and wire, and beside the splint was a tiny scratched hammer. Beto noticed it, because he noticed how things were put back together. A young woman sat there with ink on her fingers and a pencil behind each ear.

"I'm Margit," she said. "I've never seen it. I only read the letters."

She showed them a page of letters. Hundreds of them, in a line. They were the parts of a protein chain, one letter for each link.

"Some links tend to curl," said Margit. "Some tend to lie flat. This part I'd bet on."

She drew a spiral in firm black ink. Then, for the next stretch of letters, she drew a flat zigzag in light pencil.

"Ink where I'm sure, pencil where I'm not," she said.

Danya counted the turns of the spiral. Five. Eighteen links, five turns.

"Pencil means I don't know yet," said Margit.

Beto picked up the drawing and walked to the pinboard on the wall.

"Done," he said. "That's what it looks like."

A woman came out of a side room, rubbing her gloved hands. There was frost on her gloves. She carried a heavy camera case.

"Wait," she said. "I'm Kemi. A drawing is a guess until it sits beside a photograph. Let's pin them side by side."

Beto lowered the drawing. "So the drawing could be wrong?"

"It could be right," said Kemi. "We don't know yet. That's the point."

Kemi opened the case. Inside was a large grey picture.

"I took it in the cold," she said. "The real chain, frozen still, in my microscope room."

The spiral was there. Sharp and clear, five turns, just where Margit had inked it.

Then they looked at the zigzag. It was there too. But it was not alone. It lay flat, side by side with a second zigzag from further down the chain, like two sides of a folded fan.

"An accordion," said Danya.

"Where it moved, my picture blurs," said Kemi. "This part held still. So it's sharp."

Danya laid the drawing and the photograph next to each other on the table.

"How do we know who was right?" he asked. "We look at each part."

The spiral: Margit had inked it, and the photograph agreed.

The zigzag: Margit had pencilled it, and it was really there. But she had not drawn its partner.

"Show me the photograph," said Margit. She studied it for a long time. "I want to be wrong if I'm wrong. I missed the neighbour. The letters for it were far away, so I never joined them up."

"And my photograph shows what was there, not why," said Kemi. "Your letters tell us why it curls."

Danya held up the pen spring again. "So the chain curls because of what it's made of. Like the spring."

"Something like that," said Margit. "The links tug on each other."

They pinned the two pictures to the board. Drawing on the left. Photograph on the right. The first pair.

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, Beto drew his bike before he looked at it. Ink for the parts he was sure of. Pencil for the gears. Then he went outside and checked.