Chapter 18 - "Birthday Surprises" - In which Harry gives Slughorn a bezoar instead of an antidote in Potions class, then asks him unsuccessfully about Horcruxes. The sixth-years then have their first apparition lesson, where Harry overhears Malfoy saying Crabbe has been acting as his lookout, and Harry starts watching the Marauder's Map. Then, on Ron's birthday, Ron accidentally consumes Romilda Vane's love potion and Harry takes him to Slughorn to get an antidote; there, Slughorn gives Ron some poisoned mead and Harry saves his life with a bezoar.
[Harry] discusses his new homework for [Dumbledore] - obtaining the memory from [Slughorn] - first with [Ron] and then with [Hermione].
The Bezoar Solution
In [Potions] class, [Professor Slughorn] assigns students to brew antidotes to poisons, which excites [Hermione] because [Harry] won't be able to get help from the [Half-Blood Prince].
Without the help of the [Half-Blood Prince], [Harry] is unsuccessful in brewing an [antidote] to his poison. However, at the end of the lesson, he presents [Slughorn] with a [bezoar] instead - delighting [Slughorn] and infuriating both [Ron] and [Hermione].
After [Potions] class, [Harry] stays back to ask [Slughorn] about [Horcruxes] - but [Slughorn] gets upset and storms off.
[Harry] tells [Ron] and [Hermione] about his failure to retrieve the memory from [Slughorn], but they are unsympathetic thanks to his triumph with the [bezoar] in [Potions] class.
As time passes, [Professor Slughorn] stops issuing [Slug Club] invitations, but otherwise resumes his normal glowing treatment of [Harry].
[Hermione] gets frustrated when, for the first time [Harry] can remember, she is unable to find something in the [Hogwarts] [library] - the definition of a [Horcrux].
The Apparition Lesson
The sixth years have their first [Apparition] lesson with [Wilkie Twycross] in the [Great Hall]. [Harry] overhears [Draco] arguing with [Crabbe], but other than that and [Susan Bones]'s [splinching], nothing interesting happens.
After overhearing [Malfoy] and [Crabbe] during [Apparition] lessons, [Harry] is convinced that [Malfoy] is using [Crabbe] and [Goyle] as lookouts - but he doesn't know for what - and he and [Ron] run up to their [dormitory] to look for them on the [Marauder's Map].