"Why Am I Sweaty?" is a book of help for growing up that, from the front cover's blurb, is about, "your body explained in horrifyingly uncomfortable detail."
In "Carpet Diem," Grunkle Stan catches Mabel Pines (in Dipper's body) peeping into their attic bedroom while Grenda, Candy, and Dipper (in Mabel's body) start sleepover activities, and mistakenly believes that "he's" "at that creepy age where you spy on girls." He then takes Mabel to his office and forcibly has her sit and listen to him talk about puberty and the birds and the bees, using the book to explain everything. The book starts off with the pituitary gland and goes on to explain where babies come from.
"Puberty is the greatest mystery of all. Also: go outside and make friends"
- 116. "Carpet Diem"