Buckle Your Seatbelts
Most people don't know that Appa and Momo were both hopeless romantics. Both being hopeless romantics, they couldn't help but intervene in the lives of their humans when it came to matters of the heart.
Being animals, neither one was really able to understand the words the humans used. However, as animals, they did understand animalistic urges.
Like the blue female's urge to stare long and hard whenever the scarred male did his firebending without a shirt. But she would always look away blushing when he turned her way.
And the scarred one's urge to touch the blue female when she was close to him. He would sometimes reach out to her and get her attention, but just as he would make contact, he would stop and retreat. Maybe he was afraid to touch, despite his obvious urge to. His hand once touched hers and he recoiled like he just patted a viper-weasel.
The two sky creatures would watch the two from a distance, and when it seemed like the two would finally succumb to their urges and speak to one another, they would hesitate and run away, pretending like nothing had happened, despite their blushing faces. It was so damn obvious (to everyone except the humans; well, maybe not the blind one) that they urged for each other, they were just too stubborn to admit it. Needless to say, it was very fustrating.
So they decided to do something about it.
They were flying high in the air. Aang and Sokka were at the front, Toph was sitting in the back, gripping the saddle for dear life, and Katara and Zuko were sitting on opposite side of each other, too stubborn to even look at each other.
"Hey guys," Aang said. "Appa's getting tired, so we'll be landing soon."
Everyone, except Toph, got up and began stretching their muscles, sore from the long flight. That was when Momo gave Appa the signal in the form of three loud chirps. After days of waiting patiently for the right circumstances, the right moment had finally presented itself. And the flying mammals were going to take advantage of it.
Hearing the chirps, Appa stopped mid-air. The sudden unexpected stop made everyone lose their balance and falling. Katara, not even thinking, reached out for closest thing near her in an attempt to regain balance. That thing happened to be Zuko.
Pinned under Katara, the two were face to face, their noses only centimeters from one another. Zuko's hands had wrapped themselves around her waist, unconsciously trying to cushion her fall.
Tawny gold met deep blue. A single moment stretched into years where both teenagers stared deep into each others eyes, hosting thoughts and feeling both were too afraid to admit they had. The moment, that single awkward, scary, perfect moment felt like it could have gone on forever and they would lay like that forever. And for that single moment, both thought that would have been okay.
"What are you doing to my sister?" That's when reality caught up to them.
The rest of the day was full of snide remarks (Toph), angry glaring (Sokka), and completely shocked expressions (Aang). And Katara and Zuko were blushing the whole time.
But every once in a while, they would look at each other from across the room, make brief eye contact, and look away blushing, with the ghost of a smile on their lips.
Needless to say, Appa and Momo were very happy with the results.