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

It rolled into the print shop and settled on the big black press. It was Tuesday.

The newsroom door had a new hinge. Beside it, someone had scratched a tiny hammer.

Danya opened the newsroom door, and a folded note slid across the floor. Somebody had pushed it underneath.

He opened it. It said, in neat pencil:

Your bake-sale number is wrong.

There was no name.

Danya found last week's paper. Page one: BAKE SALE RAISES £430 FOR THE LIBRARY.

"Which number?" he said. "There are three numbers in that story."

Which number is wrong? See if you can work it out before the end.

Beto went red. He had typed up the bake-sale story.

"It might not even be wrong," he said. "And if it is, I'll fix it quietly. Small print, on page eight. Nobody reads page eight."

A small tidy rabbit came out from behind the filing cabinet. She had a red pen behind one ear and a green pen behind the other. Her name was Errata.

"Maybe nobody noticed," said Beto.

"Somebody noticed," said Errata. She held up the note. "And nobody trusts a paper that hides. Walk towards the mistake, not away."

"But it's embarrassing," said Beto.

"It is," said Errata. "That's why it matters."

"First, find out if it's really wrong," said Danya. "How do we know?"

A small mole in a dusty vest was already at the door, with a little lamp on her head. Her name was Deed.

"Go to the original," said Deed. "Not last week's paper. Not what somebody remembers. The cash book."

The school treasurer kept the cash book in the office. It was a big handwritten book with columns.

Deed found the page. Bake sale, Friday the 3rd. Eighty-five cakes sold at four pounds each.

Danya worked it out. "Eighty-five fours. Three hundred and forty."

"Three hundred and forty," said Deed. "The cash book doesn't forget."

Beto looked at his story. He had written 340 in his notebook. When he typed it, his fingers had swapped the three and the four. Four hundred and thirty.

"Ninety pounds too much," said Beto quietly.

"The library was going to buy books with that money," said Danya. "They planned for four hundred and thirty."

"Then they need to know today," said Deed. "Before they spend it."

Danya ran to the library with the cash book page copied out in his own handwriting.

"So we print a correction," said Danya. "On page one."

"Page one?" said Beto. "Everyone will see."

"Everyone saw the mistake," said Errata. "It was on page one. So the correction goes on page one. As big as the error."

She took out her pens. "Say what we got wrong. Say what is right. Say how it happened. Then say sorry."

Beto looked at his shoes. "I'm the one who typed it."

"The paper printed it," said Danya. "I checked the page and didn't see it. We got it wrong. Not just you."

Beto looked up.

At four o'clock on Friday, the press thumped. Page one had a box at the top with a thin black line round it.

CORRECTION. Last week we said the bake sale raised £430. It raised £340. We swapped two digits when we typed the story. We are sorry.

On Monday, a second note came under the door.

Thank you. Now I trust you more, not less.

Beto cut out the first Corrections box and pinned it to the newsroom wall.

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 dinner, Beto told his mum he had broken her favourite mug that morning and hidden it in the cupboard.

"Thank you for telling me," she said.