[Katara would know what to say. Aang felt his insides freezing up. She was only here a few days and already this place was causing her pain. He doesn't really know why death works that way here. But when he thinks of what he learns back home, an explanation comes to him.]

It's... it's because we all belong to the world we come from. When we... go, we're really just becoming part of the planet again, part of its life force. But this place... this world isn't where we belong. We're not part of this planet's life force. So once this planet realizes it... it takes a week. Then it sends the person back. But... it will take something away from the person, too. It's.. it's like going away and coming back. [Like I did..]