Though his son asks for the "truth" about the tooth fairy, a father explains that fairies used to exist, that humans and modern technology drove the fairies away, and that serious effort can make the tooth fairy real.
Gaby asks Dad (the author) to tell the truth: is there a tooth fairy or is it just parents? The author's 'truth' holds that fairies once existed, but vanished as humans gained control of their environment. Further, if one "tries really hard" to believe, the tooth fairy's voice still appears in parents' minds suggesting they provide small treasures in exchange for a baby tooth. Belief "must come from you, and you alone". Thus, Alexander urges fantasy and whim upon children, just as they are seeking reality and reason.