Here's why:
* Adventure: The series follows the Cahill family as they embark on a globe-trotting journey, facing dangerous situations, solving puzzles, and uncovering secrets about their family history.
* Mystery: The core of the story revolves around solving the mystery of the 39 clues and ultimately finding out who the true heir to the Cahill fortune is.
* Historical Fiction: Each book explores different periods in history and features historical figures and events, weaving them into the narrative.
You could also argue that there are elements of fantasy and thriller in the series, depending on the specific book.
Ultimately, the 39 Clues is a blend of different genres, creating a compelling and entertaining reading experience for young adults and children alike.