The first time he takes her flying (it's not the last), it happens like this:
Natasha looks up.
"We need to get out of here."
If she's admitting it, things must be bad, but Tony's pretty much forced to agree with her- AIM's advancing like a horde of angry beekeepers, and though between them they could probably thin them out the sheer numbers weighed against them don't look too promising.
"We really should have brought someone else."
Tony blasts a few more goons: they go down pretty fast but are replaced quickly. Natasha shrugs.
"You are the one who assured me you would be enough." she backs up a little, evaluating their surroundings, using instincts to old and well-honed that she's barely even aware she's actively using them any more- just autopilot, second nature. "Elevator shaft."
"On it." Tony's voice, made tinny by the speakers on his helmet, responds with his usual snap. Natasha always appreciates that about him: the speed at which his mind works, even if, like many fast things, it makes it somewhat difficult to control.
"You'll have to fly me up."
This gets a proper reaction- head turning, faceplate glinting dully in the light.
"Stark, we're hundreds of feet underground and my grappling line just isn't going to cut it." Natasha ducks as some laser bolts whizz past her. "I am just as thrilled at the prospect of immediate proximity to you as you are to me, trust me."
"I wouldn't say that-" And though she tenses involuntarily at the sudden contact, Stark's grabbed her and they're shooting upwards, the roar of the suit's bootjets drowning out the angry yells of AIM's goons.
"Only this isn't designed for passengers!"
It's so very, very like him to have designed a super hero suit without any concern for actually rescuing people that Natasha can't suppress a smile. Treacherously though, it turns into, honest to God, a giggle, which becomes full blown laughter as the floors rocket past. She's insanely uncomfortable, her stomach has migrated to somewhere in the vicinity of her boots, and all her finely-honed instincts are screaming danger danger danger as loud as they can, but Natasha's never been more exhilarated in her life.
Who would have guessed, she thinks, somewhere in the one part of her brain that isn't fizzing with adrenaline, that this was how Stark would finally surprise her?
It takes Tony a second to realise that the sound she's making is laughter (he's uncomfortably worried for a second that she's screaming, in which case he'd have no idea where to start dealing with it). It comes through the speakers distorted, almost inaudible against the sounds of the suit, a wild, crazy release that's actually, if he's honest, kind of attractive.
Tony wonders, for a moment, if he should say something- make a remark about power, perhaps, or arrogance, a dig at the report that was spot on in all the most hurtful ways, but in the end he simply opens up the throttle some more (so to speak), speeds up, and laughs with her.
Who would have guessed, he thinks, narrowly avoiding a jutting spear of wreckage and prompting another whoop from Natasha, that they would have this in common?
(Later, standing at the top of the shaft, they take a moment to regain their breath.
"Did Clint enjoy it that much?" Natasha asks, all guard dropped. Tony shakes his head.
And they start laughing all over again.)