Avatar: The Last Airbender, Book 3: Fire

Zuko:
Why did you do it?

Azula:
You're going to have to be a little more specific.

Zuko:
Why did you tell Father that I was the one who killed the Avatar?

Azula:
Can't this wait until morning?

Zuko:
(firmly) It can't.

Azula:
Fine. You seemed so worried about how father would treat you because you hadn't captured the Avatar. I figured, if I gave you the credit, you'd have nothing to worry about.

Zuko:
But why?

Azula:
Call it a generous gesture. I wanted to thank you for your help, and I was happy to share the glory.

Zuko:
You're lying.

Azula:
If you say so.

Zuko:
You have another motive for doing this. I just haven't figure out what it is.

Azula:
(stretching up her arms) Please, Zuko. What ulterior motive could I have? What could I possibly gain by letting you get all the glory for defeating the Avatar? Unless somehow, the Avatar was actually alive. All that glory would suddenly turn to shame and foolishness. But you said yourself that was impossible. Sleep well, Zuzu.