Here's why:
* Supernatural elements: The story involves a pair of prankster kids who hypnotize their principal into becoming the superhero "Captain Underpants". This is clearly fantastical and not rooted in reality.
* Exaggerated situations: The stories often involve over-the-top situations, like talking animals, giant robots, and absurd adventures. These are clearly unrealistic.
* Humorous tone: The series is filled with slapstick humor, puns, and silly characters. While realistic fiction can be funny, it's not the primary focus.
While the book might appeal to children who enjoy realistic fiction due to its relatable characters and themes of friendship, the core elements of the series firmly place it in the fantasy genre.