On Monday morning, Mrs Patel unlocked the classroom library trolley and stopped at once. "That is strange," she said softly. "The silver key is gone." Mina looked up from her desk. The library trolley was the favourite thing in Room 4. Every Monday, Mrs Patel rolled it to the reading rug and opened the shelves so everyone could choose a new book. Without the key, the books were trapped inside.

"Maybe it is in your pocket," said Noah. Mrs Patel checked one pocket, then the other. She shook her head. "Not today. I used it on Friday afternoon after lunch." A worried whisper moved through the class. Mina felt it too. She had been waiting all weekend to borrow the next adventure story. If the key was lost, there would be no library time at all.

Mrs Patel placed the trolley beside her desk. "We will stay calm," she said. "Think carefully. Did anyone see the key on Friday?" For a moment, no one spoke. Then Oliver raised his hand halfway. "I saw something shiny near the paint trays," he said. "But I thought it was a paperclip."

She had been waiting all weekend to borrow the next adventure story.

Mina turned quickly. The art shelf stood near the sink at the back of the room. She remembered the class rushing to wash brushes before home time. Water had splashed. Chairs had scraped. Everyone had been in a hurry. Mina crouched down and peered under the shelf. A glint of metal caught her eye. She reached into the shadow and pulled out the silver key.

"Found it!" she called. The class cheered. Mrs Patel smiled. "Well done, Mina. You solved the mystery." Mina handed her the key, feeling proud. The library trolley was unlocked, and the books were free again.