All Water Remembers the Sea

One of the first things Katara learned about bending was how the water felt to her, how it seemed to flow and slip through her fingers when she tried to get a grip on it. She kind of expected that -- like even if she scooped some out of the ocean, or melted some ice over the fire, it remembered what it was like to be part of that monolithic oneness, and refused to let a mere human child hold onto it for more than a moment.

When they took Aang north, she stood outside in her first rainfall, and, once Sokka and Aang had gone inside their shelter for the night, just stood there and felt the water falling from the sky and running over the trees and bushes of the forest, to pool in puddles on the ground. Crouching, she examined a small puddle, waving her hands over it to lift the water up with her bending -- barely enough water to cup in her hand. It hovered for a moment, a giant raindrop, before sliding through back to the ground.

Katara decided that all water remembered being a part of the sea, regardless of where it ended up later.