In the chapter of Boy entitled “The Great Mouse Plot,” we are introduced to the secret hiding place where the boys stash their “small treasures” from the prying eyes of teachers.
The boy and his four friends discover a loose floorboard at the back of the classroom. When they pry it up with the blade of a pocket knife, they find a large hollow space underneath.
Straight away, they see huge potential. From now on, this is going to be the secret hiding place for all their small treasures, such as sweets, conkers—the seeds of horse chestnut trees used in a traditional English schoolboys' game—and birds' eggs.
At the end of each lesson, the boy and his chums wait until the classroom is empty before lifting up the loose floorboard and examining their secret stash. Sometimes they will add to their hoard, and on other occasions they will take something away.
One day, they find a dead mouse in the hole. But instead of throwing it away, young Roald hits upon the idea of putting the moldering rodent in a jar of Mrs. Pratchett's sweets. Mrs. Pratchett is the owner of a local sweet shop that the boys regularly visit, and she's frequently rude and ill tempered to them. Putting a dead mouse in one of her sweet jars is a way for the boys to get back at her for all her verbal abuse.