cheek to cheek
Mr. Spock frequently observed many human customs. There were the waves (these more than just acknowledgement but even volatile indications of joy and concern in tandem), the jokes (these serving not just for humor but moreover for the increase of trust), the handshakes and other points of contact (these combining, in some sense, attributes of both of the former behaviours)...and then there were the kisses.
A kiss on the cheek was sometimes an informal gesture, sometimes a formal greeting. It had its place in diplomacy, between cultures (and, particularly, individuals of Terran origin who shared the same socio-geographical societal affiliations from whence they had come) that affirmed it so, and it had its place in friendships and other bondings even outside of those romantic relationships which were, however illogically, centred at the peak of humans' pyramid of priorities.
Between women, the cheek kisses were exchanged, and between men and women where the preexisting relationship context was one of the utmost respect and serious trust, one was occasionally seen. Mr. Spock had yet to see a significant exchange of the gesture between two masculine-aligned crewmembers, save one fleeting enthusiastic donation from Mr. Sulu to Mr. Chekov which had not been spoken of either beforehand or since. Most amusing.
All of this was mere observation, of course. It posed no issue to Mr. Spock's usual routine. Vulcans had their versions of kisses, those slight touches of the hands at nerve centres which would be denoted much drier, much more "buttoned-up" and reserved than anything Humans performed, and there was no reason to engage in any of those behaviours with anyone else present on the Enterprise.
The problem was...Ms. Uhura. While the epitome of professional excellence and appropriately-exhibited enthusiasm in her work as a communications officer, she was not without questions. Many was the time Mr. Spock had noticed her glancing forlornly in his direction when important news on the part of either of them had been exchanged, and she'd made to turn away. He suspected that she mourned the loss of the opportunity to lay a gentle hand over his or perform some other common act of human affection. This was logical, because for humans, the absence of that step meant that there was a gap in the interaction; the loop could not be closed.
Mr. Spock never proposed a solution to this quandary, however. Much as he would prefer to ameliorate the issue as quickly and smoothly as possible, he didn't quite think surrendering the sanctity of his own particular personal sensibilities was the correct or logical answer. He had only observation - he was replete with it - yet he feared no amount of observation would ever be enough to go on.
Occasionally Ms. Uhura placed a hand on his arm, and very nearly made to exert pressure. Then she would pause, awkwardly lift her hand away (the awkwardness was made apparent by the unusual number of inflection points in the appendage's departing arc), purse her lips, and raise the opposite hand to her earpiece, forgetting that she was not already seated at her console to busy herself.
She was just as hesitant as he. And it was, undeniably, important. They were not troubling themselves over trifles; for nothing.
So, at length, Mr. Spock proposed: "Ms. Uhura."
"What is it, Mr. Spock?"
"I-"
No. It wouldn't do.
He awkwardly (again, the inflection points) raised a hand halfway to his cheek, then dropped it and reattempted with the opposite hand, ending with one index finger tapped to the hollow of his cheek and one arm dangling perilously at his side. His arms and uniform shirt both felt unmanageably long.
But Ms. Uhura smiled. "I...think I see what you mean."
Gesturing gently to the upswept hand, she leant her upper body weight upon his forearm, anchored by both hands of her own, and pressed her own cheek there where he had pointed (the earpiece, on the other side of her head, stayed neatly out of the way).
Mr. Spock knew that he must still have looked quite uncomfortable.
"It's ridiculous, isn't it?" Ms. Uhura asked through a poorly-contained smile (all lips, giving way to all teeth).
Mr. Spock nodded.
"But you're ridiculous too, and so am I - so I think it's perfectly logical."
Mr. Spock kept this sweet Earth custom in mind, indeed. Whenever he was stood at the entrance to a lab or conference room, or even in front of his science station on the bridge, Ms. Uhura's presence alongside would constitute an easy signal for him to lean slightly down and to one side, that she might perch up on her boots and meet his cheek there with her own.
(Squish.)
It was highly illogical. But it made Ms. Uhura smile. Therefore, it was of an even higher purpose.