Harry Potter and the Half-Blood Prince2009

[Harry watches Slughorn's real memory, which starts with Tom tapping the hourglass]

Tom Riddle:
[approaching Slughorn] I was in the library the other day, in the Restricted Section, and I read something rather odd about a bit of rare magic. It's called, as I understand it... Horcruxes.

Horace Slughorn:
I beg your pardon?

Tom Riddle:
Horcruxes. I came across the term while reading and I didn't fully understand it.

Horace Slughorn:
I'm not sure what you were reading, Tom, but this is very dark stuff. Very dark indeed.

Tom Riddle:
Which is why I came to you.

Horace Slughorn:
[Stares at Tom for a second] A Horcrux is an object in which a person has concealed part of their soul.

Tom Riddle:
But I don't understand how that works, Sir.

Horace Slughorn:
One splits ones soul and hides part of it in an object. By doing so, you are protected, should you be attacked and your body destroyed.

Tom Riddle:
Protected?

Horace Slughorn:
That part of your soul that is hidden lives on. In other words, you cannot die.

Tom Riddle:
[Looks into the fireplace] And how does one split his soul, sir?

Horace Slughorn:
I think you already know the answer to that, Tom.

Tom Riddle:
Murder.

Horace Slughorn:
Yes. Killing rips the soul apart. It's a violation against nature.

Tom Riddle:
[Adjusting ring on his finger, the same one in present day Dumbledore's office] Can you only split the soul once? For instance, isn't seven...

Horace Slughorn:
Seven? Merlin's beard, Tom! Isn't it bad enough to consider killing one person? To rip the soul into seven pieces... This is all hypothetical, isn't it, Tom? All academic?

Tom Riddle:
[Smiling] Of course, sir. It'll be our little secret.

[Memory goes cloudy and ends]