They'd been arguing again that morning, so maybe Katara should have expected the sneak attack by her brother. He completely blind-sided her, though, tackling her into a shallow snowbank after they'd started out from the village to go fishing. His mittened hands clamped down on her wrists and held her fast. "Ha. Let's see you try and get out of this without your waterbending."
Katara struggled beneath him, but he was heavier than she was and had her arms tight against the ice. She brought a knee up against his back, but he was expecting it and leaned forward, his thick parka absorbing the weak blow. "Sokka, you jerk! Get off me!"
He brought his face close to hers, a smug grin plastered onto his features. "Just as soon as you admit that without me around to protect you, you'd be in big trouble. I mean, your little waterbending tricks are good for showing off, but it takes a warrior to–"
He never got to say exactly what a warrior was necessary for, because Katara shut him up by doing the first thing that popped into her head: she kissed him. Quick, hard, and right on the lips. Then, while he was still reeling in shock, she pulled her wrists free and shoved his chest, so he tumbled backwards off of her, blushing furiously.
"Katara! That- that's cheating! You're not supposed to--you're my sister!"
Her only reply was to bend a large pile of snow onto his head.