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. One ring.

The shed window had one new pane. In the corner of its frame was a tiny scratched hammer.

On the bench sat a big electric kettle from the café. Its cable was coiled up beside it. A note hung on the handle.

It boils and boils and never switches off. We have to watch it all day. Please help! The Café.

Beto's aunt read the note. "This one plugs into the wall," she said. "So any test with it plugged in is mine. You two only touch it cold and unplugged. Deal?"

"Deal," said Lan and Beto.

The kettle was white inside with chalky scale. Beto filled it with water and vinegar, left it, rinsed it out, and fitted the lid back on.

"Can I do the next bit?" asked Lan.

"Next time," said Beto. "I promise."

Lan had heard that before.

Beto's aunt filled the kettle, plugged it in and pressed the switch. She stood beside it. The children stood back.

It hummed. It rumbled. It boiled.

It went on boiling. Steam poured out round the lid. The switch did not click.

Beto's aunt switched it off at the wall and pulled out the plug. "Same as the note says."

Beto picked up the screwdriver. "The switch is in the base. I'll open it."

A small pink axolotl was sitting on the bench beside a thermometer. Her gills fluttered. Her name was Shift.

"The base is where the wires are," said Shift. "That's a grown-up's job. And anyway, watch the steam, not the switch."

"Why the steam?" asked Lan.

"Water gets hotter and hotter," said Shift, "until it boils. Then it stays at a hundred degrees. It does not get any hotter. The heat goes into making steam."

"So what switches it off?"

"A little metal disc," said Shift. "When steam reaches it and warms it, the disc snaps. Click. That pushes the switch off. No steam on the disc, no click."

A small pangolin in a thermal vest came over with a set of rods. Her name was Touch.

"Heat travels through what's pressed together," said Touch. "The steam has to touch the disc to warm it. So follow the steam's road. Where does it go instead?"

The kettle was cold now. Touch tapped the lid. Round its edge was a rubber ring. It was cracked all the way round.

"The steam was getting out round the lid," said Beto. "It never went down to the disc."

Beto's aunt found a new round seal in the box of spares.

Beto held it out. Then he stopped. "You do it," he said to Lan. "Not next time. Now."

Lan pressed the new seal into the rim of the cold lid with both thumbs, all the way round. She closed the lid. It shut with a snug little click.

"That's not the switch," said Beto. "That's just the lid."

"I know," said Lan. She was smiling anyway.

Beto put the old cracked seal on the shelf of fixed things.

Beto's aunt plugged the kettle in again and stood beside it. Lan and Beto watched from the other side of the bench.

It hummed. It rumbled. It boiled. This time no steam came out round the lid.

After about three minutes, there was a sharp little sound.

"Click," said Lan, before anyone else.

The switch had gone off by itself. Beto's aunt unplugged it and smiled.

Back on the Green

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

On Saturday, Lan's brother asked Beto to help him build a kite.

"Next time," Beto began. Then he put down his book. "No. Now."