Katara nodded, although she wasn't listening to what Zuko was saying so much as how he was saying it. His tone didn't sound right, to her. Her frown increased as she stepped closer, finally coming within a few feet of him. She stayed behind him, though, wanting him to be the one to face her of his own accord.
"I remember when I first started to show him waterbending," she said casually, fondness creeping into her voice at the memory even though she was really only saying it to keep the conversation going. "He caught on so fast it was scary. And he'd put these little crests on top of the waves. Or make the water do things it wasn't even supposed to do. It didn't seem fair, really."
With a shake of her head, she sighed softly and studied Zuko's back as though it was his face and she was looking him directly in the eye.
"Then I remembered that he's the Avatar, and that he's supposed to catch on quickly. How else could he hope to master all four elements?" she murmured. And, when Zuko didn't turn around at that, Katara lost what patience she had to begin with. Which wasn't a lot, really.
"Okay," she said simply, blue eyes narrowing slightly as her tone became more forceful. "What's going on, Zuko? You're not acting like yourself and it's starting to worry me."