The Vampire Diaries, Season 2

Elena:
What are you doing?

Stefan:
Katherine took a little vervain everyday and built up a tolerance to it; I could do the same with blood. I could learn to control myself on it.

Elena:
But you can't. Stefan you don't have to.

Stefan:
I almost died last night Elena, because I was too weak.

Elena:
But the last time you drank human blood...

Stefan:
I told you I'd find a way to stop Katherine, didn't I? Well this is it. This is the only way because she's stronger than me, and unless that changes, I can't protect you.

Elena:
Can we talk about this later? (points to indicate that Damon can hear them)

Stefan:
He can hear us wherever we are because he drinks this! This is the only thing that can help me.

Elena:
Are you serious or are you pretending to fight, because I can't tell if…

Stefan:
No, this is real. No more pretend.

(Elena leaves angrily)