Final answer:
In 'The Truth as Told by Mason Buttle', Mason stores his extra shirt in his school locker to manage his sweating condition.
Step-by-step explanation:
In the book The Truth as Told by Mason Buttle, Mason keeps his extra shirt in his locker at school. This detail is a part of how the character manages his excessive sweating, a condition that makes him self-conscious and contributes to his struggles in interacting with others. Mason's extra shirt is a symbolic fixture in the story, highlighting his attempts to cope with his condition and the bullying he endures.