Mac is a boy with big hands and big feet in Mrs. Jewls'sclass. In the books, he was previously named Nancy, and was very shy when he had that name. However, he traded his name with a girl named Mac from the 23rd floor, as his name is now. Nancy (formerly Mac) later became his girlfriend, and helps carry his books for him.
While at first, Mac appeared very shy, due to his former embarrasing name, ever since he traded his name, he has become very outgoing and interruptive. Often, he stops the current lesson to tell a story that is long and unrelated, which annoys the teacher.
In Sideways Stories from Wayside School, there was a chapter called "Nancy" which had him trade names since he disapproved of the one he was given, which lead to everyone else trading names causing mass confusion.