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

It rolled across the Green to the mending shed and bumped the bell on the door. One ring. Something was broken.

The bell's bracket had been mended with a new screw. A tiny hammer was scratched next to it.

A blue bike leaned against the bench with a note tied to the handlebars.

Chain very dirty. Feels wrong to ride. Thank you! Mrs Patel.

Beto turned the bike upside down so it stood on its seat and handlebars. The chain was black with old grease.

"I could take it apart," he said.

"Can I help?" asked Lan.

"In a minute," said Beto.

He wiped the chain with a rag until it shone. He fitted it back on the teeth at the front and the back. He did not ask Lan to hold anything.

Beto put on his helmet and rode the bike round the edge of the Green. It went well.

Then he stopped pedalling to roll down the slope.

The pedals did not stop. They kept going round on their own. They pushed his feet up and down, and he had to lift his legs out of the way.

He wobbled back to the shed. "It fights back," he said. "When I stop, the pedals keep going."

"The chain must be too loose," he said. He reached for the big spanner. "I'll tighten it."

A dark iron wheel sat on the bench, with teeth all round and one calm eye at its middle. Its name was Ratchet.

"Find out why first," said Ratchet. "The chain is fine. Listen to the back wheel."

Ratchet turned the back wheel slowly with one tooth. The pedals turned with it.

"Spin a bike wheel and you should hear a click," said Ratchet. "Click, click, click. That is me. Or something like me."

"There's no click," said Lan.

"Inside the back hub there is a ratchet," said Ratchet. "Little catches on springs. When you pedal, they grip, and the wheel goes. When you stop, they slip over the teeth, and the wheel runs on by itself. One way free, the other way caught."

"So why does it fight back?" asked Beto.

"Old grease," said Ratchet. "It has glued the catches. They grip all the time. So when the wheel turns, it drags the pedals round with it."

"The catch is stuck, not broken," it said.

Beto looked at the oil can. Then he looked at Lan. She had been waiting a long time.

He held the can out to her.

Lan dripped thin oil into the back hub, one drop at a time. She turned the wheel.

Nothing. She turned it again.

Click. Click, click, click.

"Listen for the click," said Ratchet.

A small brass gear with neat teeth spoke up from the shelf. Its name was Pinion. "Before she rides it," said Pinion, "count the teeth."

Lan counted. Forty-two at the front. Fourteen at the back.

"Forty-two over fourteen is three," said Pinion. "Every time the pedals go round once, the back wheel goes round three times. That is fast on the flat. But it is hard work."

"Mrs Patel lives up the hill," said Beto.

"Then use the big one at the back," said Pinion. "Twenty-eight teeth. Forty-two over twenty-eight is one and a half. Big at the back makes the hill easy."

Beto moved the chain onto the bigger cog. Lan checked that it sat on the teeth.

Lan put on her helmet. She rode the blue bike up the hill to Mrs Patel's gate. The pedals went round easily, even on the steep part.

At the top she stopped pedalling. The pedals stayed still. The bike rolled quietly, and the back wheel went click, click, click.

Mrs Patel gave them the little brass bell from her old handlebars, for the shed's shelf of fixed things.

Back on the Green

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

The next week, Beto's cousin brought a broken scooter to the shed.

Beto looked at it. Then he looked at Lan. "Do you want to hold the spanner?" he asked.