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Exactly! maybe something like that. Because I doubt that it's like the movie. The movie in my opnion, SUCKED. i don't think you plave a rock infront of your kid and if it shakes your an earthbender. then wouldn't rocks be shaking wherever you go?

In episode one, when Katara gets mad at Sokka, her bending gets out of control and she doesn't even realise it. I imagine this would be the case with any child who doesn't know they are a bender. All children get angry and cry and I'd imagine if a child is a water bender, crazy things would be happening with their tears.
Just my two cents.

Actually, Katara's mother was not a waterbender, it's a provable fact. In one episode where Katara tracked down the man who killed her mother, they showed what happened. The firebenders came to look for the last waterbender of the Southern Water Tribe - Katara - so Katara's mother lied that she was the one to protect Katara. So, if she was the last Southern waterbender while her mother was alive, then her mother couldn't have been a waterbender.
Since we're at the subject already, I too think that the element starts reacting around them at young age when they get aggressive and feel strong emotions (brings back memories of some episodes Katara was angry xd : ) 3nodding