Avatar: The Last Airbender, Book 1: Water

Katara:
Aang? Before we get to the temple, I want to talk to you about the airbenders.

Aang:
What about 'em?

Katara:
Well, I just want you to be prepared for what you might see. The Fire Nation is ruthless. They killed my mother and they could have done the same to your people.

Aang:
Just because no one has seen an airbender doesn't mean the Fire Nation killed them all... They probably escaped.

Katara:
I know it's hard to accept.

Aang:
You don't understand, Katara. The only way to get to an airbender temple is on a flying bison, and I doubt the Fire Nation has any flying bison.