The students begin classes. First is Herbology, during which Professor Sprout teaches the Gryffindors and Hufflepuffs how to collect Bubotuber pus. In Care of Magical Creatures with Hagrid, the Gryffindors and Slytherins are introduced to newly hatched Blast-Ended Skrewts, strange lobster-like creatures that bite, sting, and propel themselves by blasting from one end. Hagrid explains the differences between the male and female Skrewts. Nobody can tell where their mouths are or how to feed them, or what purpose there is in keeping them alive. Draco Malfoy complains sullenly, as usual, and Harry does his best to seem interested, so that Hagrid will feel good about his class. After lunch, which Hermione is eating once again, she dashes away to the library. Harry and Ron trek to Divination class, where the airy, cryptic Professor Trelawney predicts that whatever Harry fears will happen soon. She shows the class how to read the planets by using Harry as an example, whom she says must have been born in midwinter, because of his coloring and tragic past. He informs her that his birthday is in July. She grows annoyed that nobody takes her class seriously and assigns them a month's worth of personal predictions on the planetary alignments.
As the students enter the Great Hall for dinner, Malfoy reads aloud from an article written by Rita Skeeter, ridiculing Mr. Weasley for helping Mad-Eye Moody out of his tangle with the policemen several days before. Malfoy proceeds to insult Ron's parents, and Harry defends Ron by noting that Malfoy's mother had a perpetually unpleasant expression on her face. Harry turns around to leave, and Malfoy attacks him, the sparks of his wand just missing Harry. At this moment, Mad-Eye Moody enters the scene, swiftly turning Malfoy into a ferret and causing him to bounce in the hallway, as punishment for attacking when his opponent's back was turned. Professor McGonagall transforms Malfoy into his original form and warns Moody not to transfigure students. At dinner, the Weasley twins speak in awe of Moody's teaching abilities in defense against the dark arts.
Over the next few days, Harry notices that Professor Snape seems especially timid and innocuous around Professor Moody. Snape is a nightmare of a teacher to everybody. He forces Neville Longbottom to spend an afternoon disemboweling horned frogs. The Gryffindors excitedly await Moody's class, and finally Thursday comes and they go to his classroom. He hobbles in, grizzled and gruff, and he tells the students to put away their books; he says that the class has studied dark creatures but that they were behind in curses, and that he has one year to cover the subject. He adds that instead of teaching just the counter-curses, he and Dumbledore agreed that it would be best to teach the curses themselves, in order to show the students what they are up against. He uses three spiders to demonstrate the three unforgivable curses, each of which would land the curser a lifetime in Azkaban. He first demonstrates the Imperius Curse, which allows the curser to control the cursed. This is the curse that Voldemort used to put wizards under his control). He then shows the Cruciatus Curse, which causes terrible writing pain to the cursed, and which upsets Neville tremendously when he sees it done to the spider. Finally, he shows Avada Kedavra, the killing curse, which only Harry Potter has ever survived. Harry is upset to see the final curse, which killed his parents; it kills instantly, yet it leaves the victim entirely unmarked. Moody warns them all to keep "constant vigilance!" and he dismisses them from class. After class, Neville is still shaky from watching the second curse. Moody pulls him aside and gives him a book on Mediterranean water plants, saying kindly that Professor Sprout has mentioned that Neville was good at Herbology.
That night, Harry and Ron sit in the common room doing their Divination homework, making up horrible scenarios that can happen to them on account of the planetary alignment. The Weasley twins are sitting quietly in a corner poring over a mysterious piece of parchment. Hermione enters the common room with a box of badges, each bearing the letters S.P.E.W. The acronym stands for the Society for the Protection of Elfish Welfare, an organization she has just begun. Before Harry and Ron can protest wearing the pins, Hedwig enters the common room with a letter for Harry from Sirius, which says that he has heard of many odd signs and wants Harry to go to Dumbledore if the scar hurts again. He says that he is returning and will be in touch. Harry finishes reading and feels like a hypochondriac. He thinks that he has placed Sirius in danger by telling him about the scar. He unhappily goes to his dormitory, not noticing that Neville is also awake and restless.
Moody is the most formidable teacher Harry has ever had; although it is widely known that Dumbledore is a powerful wizard, Harry has never actually watched him in action. Moody knows and performs the most dangerous and difficult spells, and he lives up to his reputation as being as eccentric as he is skilled. He is kind to the good characters in this book, including Neville, whom other teachers often ridicule; people at Hogwarts act respectfully toward him because they are aware that he could turn them all into ferrets if they don't. This respect even extends to Snape, who usually acts insolent to the defense against the dark arts teachers, as he is known to covet their teaching position. Hagrid's description of the sexual difference between the Skrewts highlights Harry's growing awareness of sexuality. In previous novels, differences in sex were never really considered; now, as the characters are entering a phase of adolescent crushes and sexual tension, the distinction is being made more clearly, as it is beginning to matter in the lives of the characters.
Hermione is pursuing her cause seriously. Rather then objecting to the subjugation of house-elves by immaturely not eating or simply stomping her foot, Hermione has taken agency to create a society to protect the rights' of the house-elves. This is yet another example of her growth throughout the series; J.K. Rowling did explain in interviews that these seven books would trace the growth of the characters, and in this book, Hermione stands out as the closest to adulthood-here with her determination, and later with her calm unwillingness to take sides when Harry and Ron are not getting along.