Here's why:
* Age Range: It's written for children between the ages of 8 and 12, which is the typical range for middle-grade fiction.
* Themes: The book deals with themes like friendship, family, secrets, and adventure, which are common in middle-grade literature.
* Plot: The story involves a young protagonist who gets involved in a mystery or adventure, which is a typical middle-grade plot structure.
Although it has elements of mystery and adventure, it's not classified as those genres specifically.