Push the limits
It had been completely unexpected, but Zuko was coming to learn that in this particular group, unexpected happenings were considered normal. He would almost hate to find out what it was like when expected things happened. He figured that if he found that out, then it would mean the world was ending and it was time to trade in his broadswords for a hole in the ground in hopes that the ending would pass him by.
It had started like one of their usual arguments, though truth be told he could no longer remember what the original spark had been. Probably something stupid again, like the time an argument arose over the fact that he was so obviously not a morning person. Katara had stated that he made Early Morning Sokka look pleasant by comparison, which hadn't gone over great as Zuko and Sokka got along oh-so-well with each other. So of course a heated argument sprang up between the fire and water bender, and things pretty much stayed in that vein for several weeks.
So when yet another argument broke out between Katara and him, no one in the group had even blinked, and continued on with their original tasks. Toph had dragged Aang off to work on his earth bending, Iroh had wondered off with Sokka to do some fishing, and Appa and Momo had settled in for a nice nap. He hadn't noticed anything out of the ordinary himself, just the usual routine argument. He had just worked himself up into a good rant when Katara sighed, rolled her eyes, and grabbed his upper arms. She effectively silenced him in the age old tradition used by countless people to put an end to diarrhea if the mouth by pressing her lips against his own.
The kiss lasted barely a minute, but by the time Katara pulled away with a triumphant smirk on her face, Zuko was so shocked that he was rendered completely mute. His eyebrow was somewhere around his hairline as he stared at her. She simply shook her head and walked away, muttering something about how for once her brother's advice to Aang had come in handy.
He was still standing frozen in place when Iroh and Sokka returned from their fishing expedition. Sokka shook his head, chalking Zuko's strangeness up to the fact that it was Zuko. Iroh, on the other hand, figured that something had happened, though no amount of badgering on his part could get the story from his nephew.