Christopher describes photographs taken in 1917 that appear to show live fairies. The incident was dubbed “The Case of the Cottingley Fairies,” and Sir Arthur Conan Doyle, author of the Sherlock Holmes stories, endorsed the photographs as proof of the existence of fairies. In actuality, the fairies shown were just cut outs, which the photographers admitted in 1981. Christopher explains Occam’s razor, a law that says “no more things should be presumed to exist than are absolutely necessary.” To Christopher, this means that murderers tend to know their victims, fairies don’t exist, and you can’t talk to the dead.
At school, Siobhan asks Christopher why his face is bruised. He explains the fight with Father. Siobhan reluctantly accepts that nothing happened worth worrying over, largely because Christopher cannot remember whether or not Father hit him.
Christopher returns from school before Father gets home from work. Christopher gets the key to the garden from the china pot and sets out to retrieve his book. When he doesn’t find his book in the trashcan, he realizes Father might have hidden it elsewhere in the house. He eventually discovers the book in a shirt box underneath a toolbox in Father’s bedroom closet. Though happy that Father hasn’t thrown his book away, he worries that Father will know he has been searching through his things. Just then, Father comes home. Christopher rushes to put everything back the way he found it. At the last moment he notices a letter in the shirt box addressed to him and sees there are several such letters. He takes one of the letters, puts his book back, and tiptoes back to his room.
Later, Father makes dinner, then begins to set up shelves in the living room. Christopher uses the opportunity to read the letter alone in his room. The letter is from Mother, and it describes a new job she has working as a secretary in a factory. She tells of the apartment that she has moved into in London with a man named Roger and notes that she has not received any letters back from Christopher. She says she loves him very much anyway. The letter confuses Christopher because Mother never worked at a factory or lived in London. The letter has no date, but on the envelope it bears the postmark “16th of October 1997,” eighteen months after Mother died. Christopher feels excited to have a new mystery on his hands, but he decides not to jump to conclusions. He hides the letter and goes downstairs to watch television.
Christopher describes scientific mysteries as problems that have yet to be solved. Christopher says ghosts are only a mystery because we do not know the science behind them, but one day we will. He talks about the seemingly random number of frogs in the pond at school from year to year and notes that a formula, discovered by a group of scientists, shows that the population density of frogs runs in predictable cycles that only appear random. Christopher concludes that sometimes complex problems follow simple rules, and that whole populations can die out for no reason other than the way the numbers work.
Christopher’s discovery of the letter from Mother adds a dramatic twist to the plot and reveals another secret that Father had kept from Christopher. The letter initially confuses Christopher because it contains facts about Mother’s life working at a factory in London, although to Christopher’s knowledge Mother had never worked at a factory or lived in London. The postmark dates the letter to several months after Mother’s supposed death, leading Christopher, who evidently doesn’t consider that Mother might not be dead, to question how this scenario could have occurred. He wonders if perhaps the letter was in the wrong envelope, and he even speculates somewhat comically that it could be a letter to a different Christopher from that Christopher’s mother. The reaction creates an irony in which the reader, unlike Christopher, realizes that the letter suggests Mother never died at all, and that Father has likely been lying about this fact as well. Before Christopher considers this possibility, however, he decides not to jump to conclusions without more information, implying that he will soon undertake a new investigation.