If anybody understands what powerful and long-lasting magic causes this book and quill to behave as they do, nobody has ever confessed to it, doubtless because (as Albus Dumbledore once sighed) it saves the staff tedious explanations to parents who are furious that their children have not been selected for Hogwarts. The Book and Quill’s decision is final and no child has ever been admitted whose name has not first been inscribed on the book’s yellowing pages.
At the precise moment that a child first exhibits signs of magic, the Quill, which is believed to have been taken from an Augurey, floats up out of its inkpot and attempts to inscribe the name of that child upon the pages of the Book (Augurey feathers are known to repel ink and the inkpot is empty; nobody has ever managed to analyse precisely what the silvery fluid flowing from the enchanted Quill is).
Those few who have observed the process (several headmasters and headmistresses have enjoyed spending quiet hours in the Book and Quill’s tower, hoping to catch them in action) agree that the Quill might be judged more lenient than the Book. A mere whiff of magic suffices for the Quill. The Book, however, will often snap shut, refusing to be written upon until it receives sufficiently dramatic evidence of magical ability.
Thus, the very moment that Neville Longbottom was born, the Quill attempted to write his name and was refused by the Book, which snapped shut. Even the midwife who attended Alice Longbottom had failed to notice that Neville managed to shift his blankets more snugly over himself moments after birth, assuming that his father had tucked the baby in more securely. Neville’s family persistently missed faint signs of magic in him and not until he was eight years old did either his disappointed great aunts and uncles, or the old stickler of a Book, accept that he was truly a wizard, when he survived a fall that should have killed him.
In fact, the Book’s sternness has a purpose: its track record in keeping Squibs out of Hogwarts is perfect. Non-magic children born to witches and wizards occasionally have some small, residual aura of magic about them due to their parents, but once their parents’ magic has worn off them it becomes clear that they will never have the ability to perform spells. The Quill’s sensitivity, coupled with the Book’s implacability, have never yet made a mistake.