During private meetings held throughout the year, Dumbledore uses his Pensieve to show Harry memories of Voldemort's past. A memory Harry procures from Slughorn confirms Dumbledore's suspicion that Voldemort splintered his soul into seven fragments in order to achieve near immortality. Six of these fragments are contained in magical objects called Horcruxes, which must be destroyed before Voldemort can be killed. Two Horcruxes have already been destroyed: Tom Riddle's diary, which Harry stabbed with a basilisk fang in his second year, and Voldemort's grandfather's ring, which Dumbledore destroyed during the summer. Dumbledore suspects that the remaining Horcruxes are Voldemort's pet snake Nagini and objects formerly owned by Hogwarts' founders: Salazar Slytherin's locket, Helga Hufflepuff's cup, and an unidentified object of either Godric Gryffindor's or Rowena Ravenclaw's.
After Snape sees Harry cast a curse from the Half-Blood Prince's book and attempts to confiscate the book, Harry hides it in the Room of Requirement. Harry's Hogwarts House, Gryffindor, wins the school's Quidditch championship; euphoric, Harry spontaneously kisses Ginny, and with Ron's diffident approval, they start dating.
