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

Danya was making salad dressing in the community kitchen. Oil, vinegar, a pinch of salt. The jar's tin lid had been dented once and tapped round again, and on its rim was a tiny scratched hammer. He shook the jar hard. The oil broke into a hundred tiny drops.

Then he set it down and watched. The drops crept together. In a minute they were big drops. In five, the oil sat in one layer on top.

"Why do they find each other?" asked Beto.

"They don't find anything," said Danya. "But something puts them together. How do we know what?"

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

Margit had a page of letters, and some of the letters were circled.

"These links are oily," she said. "Water doesn't mix with them."

Danya looked. "Then they go on the outside," he said. "So they can touch the oil in the cell."

Margit shook her head gently. "Think where the water is," she said. "A protein floats in water. Water clings to water. The oily parts get crowded together, away from it."

"Like the drops in my jar," said Danya slowly.

Beto picked up a pencil and drew a jar of water. He drew little oily dots scattered through it. Then he drew the same jar a minute later, with the dots in one clump in the middle.

"The water closes up around them," he said. "So they end up together."

"That's it," said Margit. "Nobody pushes them. The water just holds on to itself."

Margit counted the circled links. Twelve.

"This part I'd bet on," she said.

She drew the folded chain with the twelve oily links packed into the middle, like stones in a peach. She put the water-loving links around the edge.

"Ink where I'm sure, pencil where I'm not," said Margit. This time, almost all of it was ink.

Kemi came out of the cold room. Her gloves were white with frost.

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

The photograph showed the fold. In the middle, oily links, packed tight. Danya counted them. Eleven.

"There should be twelve," said Beto.

Kemi pointed to the edge of the picture. One oily link sat on the surface. It was pressed flat against something else: the edge of a second protein.

"My photograph shows what was there, not why," said Kemi. "You'll have to work out why."

Beto frowned at the picture. "So Margit was wrong about one?"

"Show me the photograph," said Margit. She looked closely. "I want to be wrong if I'm wrong. But am I?"

Danya saw it first. "The outside one isn't touching water," he said. "It's touching the other protein. So it's still out of the water. Just in a different way."

"Where they meet is dry," said Kemi. "That's often where two proteins hold on to each other."

Margit laughed. "So my rule was right, and my drawing missed the partner. I only drew one protein."

"A blur tells you something too," said Kemi. "And so does a neighbour."

Beto counted again, to be sure. Eleven inside, one at the meeting place. Twelve.

They pinned the third pair to the board. An inked core on the left. A photographed core on the right, with one link at the edge where the partner had been.

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 tea, Beto poured a drop of oil onto his soup. He stirred it into tiny drops and watched them gather. "Crowded out by the water," he said.