Chapter 23 - "Horcruxes" - In which Harry learns that Dumbledore has returned, races off to his office, and with him watches Slughorn's memory, where Riddle learns what Horcruxes are and contemplates making more than one. Dumbledore explains his suspicions that Riddle made six, including the now-destroyed ring and diary, the locket, the cup, and Nagini. Dumbledore promises Harry can join him in destroying the next one, and explains to Harry what love - and the prophecy - really mean.
One More Try for a Memory
Harry Uses Felix Felicis to Great Effect
Harry takes a swig of [Felix Felicis] and with full confidence decides to head down to Hagrid's [hut], alarming Ron and Hermione. As he leaves Gryffindor Tower, he hears Lavender accosting Ron and Ginny chewing out [Dean Thomas], putting him in an even better mood.
Harry walks back into the castle with the newly obtained memory and heads to Gryffindor Tower, but the Fat Lady pretends the password has changed and won't let him in. When Nearly Headless Nick informs Harry that [Dumbledore] has returned, though, he heads off to Dumbledore's office instead.
The Real Memory