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The next day in Potions class, the Half-Blood Prince lets Harry down at first by not providing any hints on how to make an antidote for a poison. However, as an alternative, the Prince notes that you can just use a bezoar to cure a poisoned person.
Slughorn is charmed by "Harry's" cheeky bezoar solution, but still thinks Harry should learn to brew an antidote potion.
After class, Harry corners Slughorn and asks the Potions professor about Horcruxes.
Slughorn quickly clues in to the fact that Dumbledore has sent Harry to collect his Horcrux memory. He nervously tells Harry that there is no other part of the memory, and flees the classroom as quickly as possible.
Harry realizes that getting that memory is going to be way harder than he thought.
Later on, the sixth years have their first Apparition lesson in the Great Hall. Apparating proves to be very, very tricky, and no one seems to be able to do it at first.
Harry overhears Malfoy talking to Crabbe, telling him "it's taking longer than I thought" and instructing him and Goyle to continue to keep "a lookout" (18.77).
Harry is immediately suspicious and begins to check his Marauder's Map religiously for signs of Draco's whereabouts. To Harry's confusion, Draco often doesn't show up on the map.
Soon after, Ron's birthday arrives. As Ron explores the stack of presents next to his bed on his birthday morning, he stumbles across a box of chocolates and begins eating them.
Unfortunately, the chocolates were Harry's chocolates that he had pulled out of his trunk while retrieving the Marauder's Map.
The chocolates contain a love potion that Romilda Vane cooked up for Harry when she wanted him to ask her to be his date to Slughorn's holiday party.
Ron falls fast and hard for Romilda. Because it's been a few months since the potion was concocted, it's only gotten stronger.
Harry decides to take Ron to Slughorn's office immediately, before Ron can do any damage.
Slughorn, though still wary of Harry, agrees to help. He gives Ron an antidote.
Ron begins to feel better and drinks some mead that Slughorn gives him.
Ron immediately begins to twitch and convulse; he's been poisoned.
Harry recalls the Half-Blood Prince's tip on poison antidotes and thrusts a bezoar into Ron's mouth.