Harry Potter & the Half Blood Prince SPOILER ALERT!
Chapter Seventeen: A Sluggish Memory
Synopsis
Harry, Hermione, Ron, and Ginny return to school. They find out that the Ministry is offering Apparition lessons to sixth years. They sign up for the lessons.
Harry has his third lesson with Dumbledore. They discuss Tom Riddle's first year at Hogwarts; where he has started to form a 'group of friends' who later become the first Death Eaters.
They use the Pensieve to view two memories.
Memory #1 (Morfin): Harry discovers how Voldemort killed his Muggle Father and Grandparents.
Memory #2 (Slughorn): Tom Riddle was a student of the potions professor. Dumbledore tells Harry that this memory is most important. Harry finds out that Voldemort has tried to ask Slughorn about Horcruxes. However, Dumbledore tells Harry that the memory has been tampered with. Dumbledore says Harry must persuade Slughorn to reveal Slughorn's true memory of this incident.
Theory: Hmmm, I think Slughorn actually told Tom Riddle about Horcruxes. How? TR must have been able to use Veritaserum on the potions professor.
Rambling Gypsy