It is just past noon. They, all ten of them including the animals, have left the Western Air Temple at last, feeling it was time to move onward in their journey.

It is also wretchedly hot. The sun is just past its zenith, and though it is partially obscured by clouds, the air is moist and oppressive. The humidity makes all of them miserable, and in this heat Appa gets tired far too quickly, especially with eight people to carry. For once, no one complains when they are all forced to travel by foot (or in Teo's case, by wheel) in order to give the weary skybison a break.

When Aang suddenly stops in midstep and stares up at the sky, they all stop and stare at him.

"Aang?" says Katara worriedly.

Aang does not answer.

--

In his mind, Aang is remembering a day just like this one, over a hundred years ago. He and Monk Gyatso had been visiting the Western Temple together, and they had gone on a walk through the plains above the temple valley. Aang had almost tripped a dozen times, preoccupied by the heartbreaking blue of the sky above them.

Gyatso finally clucked his tongue at him. "Why so clumsy, Aang?"

Aang flushed. "Sorry, Monk Gyatso," he mumbled. "It's just—I keep looking up at the sky. It's really blue today. It's kind of pretty."

Gyatso shook his head. "No, Aang," he said gently. "The sky is not blue."

Aang stared at his guardian, perplexed. "It's not?"

The wizened monk tipped his face upward. "The sky," he murmured, "is a limitless thing, transcendent, defying all existential recognition. It is neither blue nor any other color. It is beyond color, beyond light, beyond description. 'The sky' is an appellation we give to the expanse of vision we see above our heads, above our earth and water. 'The sky' is not a concrete thing, nor is it abstract. And it is certainly—" he lets out a small chuckle, "—not blue."

Aang looked up too, frowning. "Then—why do I see blue when I look at the sky?"

"A better question to ask," Monk Gyatso said softly with his customary patience, "would be, 'What do I see that is so blue?'"

Aang thought about that, his eyes tracing invisible paths through the swath of blue over his head. "The air?" he guessed finally.

"Yes!" Monk Gyatso praised him. "Very good, Aang. The air is, indeed, very blue today. It is rather pretty, I agree."

Aang blinked. "Uh. Right."

Gyatso laughed softly. "You will understand someday," he promised.

"Uh…huh." Aang frowned at his teacher. "If the sky isn't a concrete thing or an abstract thing, is air concrete or abstract?"

Delighted, Gyatso smiled approvingly. "You are beginning to think like a sage," he said.

Aang scratched his head sheepishly. "Uh, thanks, Monk Gyatso, but—that doesn't really answer my question."

"I cannot," admitted Gyatso frankly. "It is your question. It is for you to find the answer, Aang."

"Right," said Aang again, a little annoyed. He glanced away, over the plains.

Gyatso regarded him thoughtfully. "Think, Aang," he said encouragingly. "Is air a tangible thing? Can you feel it, hear it, taste it?"

"Well," said Aang slowly, his brow furrowing. "Not…really, I guess. I mean, if you're standing still and not moving, you don't feel it. Unless it's really windy or something…"

"Precisely," said Gyatso, waving a finger at him for emphasis. "What is the wind, Aang?"

"Moving…air," Aang said hesitantly.

"And what moves the air?"

"Bending?" suggested Aang with a grin.

Gyatso chuckled. "Besides bending, Aang."

Aang shrugged. "Something about the sun, I think. I don't really remember. Those lessons are always so boring."

Gyatso grandly ignored the complaint. "Yes, you are correct. The heat of the sun moves the air. Now, when we airbend, what are we doing?"

"Moving air around—wait," Aang said suddenly, his eyes wide on his teacher's face. "Are you saying that airbending isn't really moving air—it's moving heat?"

Gyatso smiled at his pupil proudly. "You are indeed a very bright young boy," he said. "Yes, Aang. Airbending is the manipulation of our body heat in order to create air currents."

"Wow." Aang, looking thrilled at figuring it out on his own and awed by the revelation, was lost in thought. "I always thought it was just like—whoosh." He made a hand motion. "You know? And then—ta da! Airbending."

Gyatso tilted his head to one side. "Strangely, I do think I know what you mean," he said wryly. Aang grinned up at him, then squinted up at the sun for a moment.

"Body heat makes airbending, huh?" He switched his gaze to the blue sky again. "Uh, Monk Gyatso?" He peered at his guardian. "What does this lesson have to do with why the sky is—or isn't—blue?"

Monk Gyatso's eyebrows rose. "Lesson? I don't know what you're talking about, Aang," he said mildly. "You were the one who asked about the sky."

Aang rolled his eyes. "I still don't understand that part," he said. "And you never told me whether air is concrete or abstract."

"No, I didn't," agreed Monk Gyatso lightly. "Now, what have you learned?"

Aang sighed in mock resignation and folded his hands behind his head, reciting obediently, "The sky isn't blue, and it's not concrete or abstract. The air is what's blue, somehow, I guess, and it's moved by the sun's heat. Airbending is the manipulation of body heat. Anything else I need to know?" He grinned cheekily.

Gyatso seemed to consider the question. "Well. I could tell you about the further manipulation of light and its importance in airbending—"

Aang groaned comically.

"—But I think we should probably just continue our walk," Gyatso finished, winking at him.

Aang grinned. "Good idea."

After they'd continued their walk for a few minutes, Aang said tentatively, "Monk Gyatso?"

"Hmm?"

"I'm still not sure what you mean by the whole sky thing. But…if the sky's not really blue—"

"Yes, Aang?"

"…Is the grass really green?"

Gyatso laughed.

--

"Aang?"

He blinks, coming out of his reverie, staring into Katara's worried face. "Uh—what?"

"Is something wrong?" she says. Concern is evident in her voice and in her eyes. How long has he been standing there, he wonders, just looking up at the sky?

"No," he says quickly with a reassuring smile. "Everything's okay. I was just…thinking about something."

"Well, what is it, Airhead?" says Toph impatiently. He thinks that there is a trace of worry in her voice too, despite the nonchalant, bored tone. It makes him want to smile.

He looks up again. "Just…the air is really blue today," he murmurs softly. "It's really pretty, don't you think?"

Sokka says slowly, "Uh…you mean the sky, right?"

Aang shakes his head and lets the smile curve his lips. "Nope. I mean the air." He looks at Appa. "Right, buddy?"

Appa, despite the humidity and the heat, raises himself onto his two hind legs and waves four massive paws in the air, lowing gently at the clouds.

"Yeah," laughs Aang. "I know exactly what you mean."

--

A/N: Why, yes, that was a bunch of BS, and I had no idea what Gyatso meant by the air being blue either. It is beyond simple human comprehension. You have to be a ninety-year-old Air Nomad to get it. (glances furtively around) Of course that's it. And aww, isn't it cute that Aang can understand Appa-speak but not monk-speak? I think he's around ten in the flashback. X3 D'AWW AANG.