Your child is the detective in their own hometown.

Thursday came, and with it, gymnastics practice. Nora had no plan in particular that day, only her detective's promise to notice everything. She had spent two whole days turning the mysterious letter over in her mind, reading it before bed and again at breakfast, and she was no closer to understanding it. But it turned out she didn't need a plan. She had Scout.

Scout came along as always, padding to his corner to lie down like a gentleman. He watched the kids tumble and the chalk dust drift, and he waited, the way a good detective's assistant waits. But partway through practice, while the other kids worked on the balance beam, he stood up. His nose was twitching. His ears had gone alert. Something had caught his attention over by the big blue tumbling mats that leaned against the far wall.

Nora watched him pad over to the mats, sniffing harder and harder, his whole body suddenly serious. They were taller than she was and heavy, and they leaned against the wall with a narrow dark gap behind them, a gap nobody ever bothered to look into. Scout pushed his nose into that gap and froze, tail going up like a flag.

When Coach Dana was busy spotting someone, Nora slipped away from the group and hurried over, her heart beginning to thump. "What is it, boy?" she whispered, kneeling down beside him. "Find it."

Scout pushed his nose deeper into the gap behind the mats and snuffled along the floor. His tail began to wag faster and faster. Then he let out a muffled "Rrrooowl!" (muffled, because his mouth was full) and backed out with something held gently in his teeth, dropping it into Nora's hand before adding a triumphant "WOOF! WOOF!" just in case she had missed how clever he was.

It was a small notebook, much smaller than hers, with a cover of shiny silver. The corners were soft and worn, as if it had been carried in a pocket for a long time. Some of its pages had been torn out, leaving little ragged edges along the spine. But on the very first page, in slanty blue handwriting that Nora recognized at once, were three lines.

Her breath caught. The handwriting was exactly the same as the mysterious letter from the park. The same slant. The same blue ink. The same careful, worried loops on the letters.

"It's the same person," she whispered, hardly daring to breathe. "Whoever hid this notebook is the same person who wrote that letter. And they hid it back here, behind the heavy mats, where they thought nobody would ever look." She glanced at Scout, who was wagging proudly. "Nobody except a Wonder Dog with a very good nose."

She read the three lines on the page. The first said: They will not listen to me, so I have hidden the proof where it is safe. The second said: Read the numbers the way I taught you, using my letter. The third was not words at all. It was a row of numbers:

"It's a code … but an encrypted code of some kind," she said slowly, and a shiver of excitement ran all the way down to her sneakers. "Scout, it's a secret code. And I think the crumpled up letter in my pocket is the key." A code meant somebody had something to hide, and something to hide meant her mystery was real, every bit of it.