Yuma helped in the school library every Wednesday after class.

He liked the quiet room, the smell of old paper, and the small jobs that made the library work.

That afternoon, rain tapped on the windows, and most students had already gone home.

Ms. Kato, the librarian, looked at the wall beside her desk and frowned.

"Yuma, have you seen the small brass key?" she asked.

"The key for the reading room?"

"Yes. I always hang it on this red hook. But it is gone."

The reading room was a small room behind the library. Students used it for group work and club meetings.

"We need it tomorrow morning," Ms. Kato said. "The first-year students will visit the library."

Yuma checked under the desk and behind the computer. The key was not there.

Then a girl near the art books stood up. She was Nao from the art club.

"I can help," she said. "Our posters are in the reading room, so I want the key found too."

Ms. Kato nodded. "Thank you. Please check only safe places. Do not open any locked doors."

Yuma and Nao began with the lost items box. It held a glove, two pencils, and a blue umbrella cover, but no key.

Next, they checked the copy machine. Nao found a tiny piece of red string on the floor.

"Does the key have red string?" she asked.

"Yes," Ms. Kato said from the desk. "I tied it to the key yesterday."

Yuma looked at the floor. The string led toward the return cart.

On the cart, several books waited to go back to the shelves.

Yuma opened the top book. Nothing.

Nao opened the second book and smiled.

A brass key lay between two pages, like a strange bookmark.

"Found it," she said.

Ms. Kato came over and let out a long breath.

"Oh, I remember now," she said. "A student returned this book during lunch. I held the key while I checked the book. I must have put it inside by mistake."

Yuma laughed. "So the library key was borrowed by a book."

Nao looked at the cover. The book was called Small Doors Around the World.

"That is a perfect place for a key," she said.

The next morning, the reading room opened on time.

Nao's posters were on the wall, and the first-year students looked around with bright eyes.

Ms. Kato put the key back on the red hook.

This time, she added a large label: READING ROOM KEY.

Yuma looked at it and smiled.

In a library, even a small key could start a story.