Why does Katara say "It's not magic. It's waterbending!" to Sokka

It always struck me as a pretty strange line in retrospect.

From the way Katara is talking to Sokka you would think he had zero idea what waterbending is. I get that it's an important exposition dump for the audience but re watching it, it strikes me as a strange line like surely Sokka would know what waterbending is, he's not gonna think Katara is performing some kinda strange magic act.