Disclaimer: Not mine!
It starts on a field in Kansas.
He'd been aiming for Topeka the planet, but had landed them instead in a place that was almost—but not quite—as far away from Topeka, Kansas as one could possibly be while still, y'know… actually being in Kansas. The embarrassing affair had put him off, and though they'd managed to salvage the day by saving a rural family from an infestation of Krrrvlllchks, his mood as they trekked back to the TARDIS was decidedly foul.
"Why don't we stop off a bit before we leave," she suggests, pulling him towards the long grass off the side of the road. "Look at the clouds or something. Kip in the sun. Could be fun." It's not the best idea she's ever had, but she doesn't much fancy the thought of getting stuck in the Vortex with a grumpy Doctor.
"How boring are you, Rose Tyler?" he scoffs with a roll of his eyes, and she suddenly thinks he'll be lucky to even make it to the Vortex, at the rate he's going.
She raises a cool eyebrow at him and stops walking, jerking him back by their linked hands. "Sorry, what?"
"Anyone and their brother can cloud gaze on their back, looking up at the sky. How many people can say they've done it looking down?"
Which is how they end up floating in the air somewhere over America with the TARDIS door wide open, feet dangling over the edge, racing each other to find the most interesting formations. (He wins, but only by cheating and claiming alien shapes she'd have no way of knowing.)
"Don't worry about it," he shrugs, when she calls him out on it. "Time I'm through with you, you'll be seein' Raxicoricofallipatorians with the best of 'em."
She beams at him. "I'd have to learn to say it, first."
"I can teach you that, too."
"That a promise, Doctor?" she asks, eyes dancing with a challenge. "Keep travellin' til I can beat you at cloud gazing?"
He smiles the slow-growing smile he saves just for her. "It's a promise. And if you're very lucky, I'll even let you stay a bit longer than that."
And just like that, a tradition is born.
He never makes a habit of it—never sets aside a specific day or time. Every game is a surprise, and every game is a welcome calm between bouts of running for their lives. A chance for them to sit still, even if just for a few moments, and reconnect.
So eight months later when he goes and changes his face on her, it's not really a shock that he takes her up to play three separate times before they've even hit New Years.
(She lets him win those ones, just in case.)
"Dragon," she decides, and he shakes his head fondly.
"That's not a dragon, that's a Cnys'th. Look closer—see the ridges on the spine and the forked tail? I should take you to Burnus in the 17th century; Cnys'th roam whole continents in flocks." He pauses; frowns the tiniest bit. "Murders? …Gaggles? No, flocks."
"Are they dangerous?" she asks, trying to get him back on track.
He laughs. "Not at all! Weeeell, maybe a little. If you provoke them. Or… make eye contact. On second thought, perhaps Burnus in the 34th century would be better. You can feed domesticated Cnys'th on wildlife reserves, by that point. They become an endangered species in 3289. Oooh, rubber ducky!"
"Yes, look—there. Just there. See?"
"I don't see a rubber ducky. Just a hea—"
"—no, no, you're not looking properly. Tilt your head to the left a bit. See his wee little wing? And his beak, sticking out?"
"Oh! Yes. Very cute. But… I saw something else."
"It's swimming towards water, Rose, what else could it be?"
"A… a heart?"
Degree by slow degree, he tilts his head back up to the right. A look of utter amazement blossoms on his face.
"Oh," he says quietly, and she doubts he's ever been more breathtaking than he is at this moment.
A universe away and a lifetime later, she lands a zeppelin on their front lawn.
"Anyone and their brother can cloud gaze on their back," she quotes with her tongue in her teeth, when he runs out to investigate—shouting and flapping his arms about, still wearing his jim-jams. "But how many can say they've done it looking down?"
He interprets done it rather loosely, in her opinion; by the time they're airborne, they find they have very little interest in looking at the clouds after all.