It happened by accident.

The Enterprise had been exploring a newly discovered planet which happened to be undergoing a transition similar to the Jurassic period of Earth, down to the animal life, which were not unlike dinosaurs. Despite the ferocity of the more carnivorous species, there were surprisingly minimal casualties, and in fact the landing party had actually managed to kill (unfortunately, in Mr. Spock's opinion, as it disregarded the sanctity of life, but, he realized, necessary to preserve theirs) a type of large feathered theropod which attacked them. They had brought the body to the science lab for further examination, and there the chief medical officer and science officer had gone to work while waiting for whatever inevitable catastrophe would probably afflict the landing party any minute.

Spock and McCoy had performed the autopsy and been studying and analyzing the different components in unusual, peaceful silence for at least twenty minutes-a new record for them. Spock wondered if he should alert the doctor of this fact, but decided it would cause McCoy to make a, he had heard the captain use the term "snide" comment, along the lines of there being a first time for everything, or perhaps that it wasn't so bad working with a computer that was being used for its intended purpose, or in some way bring things back to their typical, argumentative status. And while he did not find these times unpleasant anymore...Spock appreciated the occasional moment of peace between them.

After the examination was complete it was necessary to remove the long tail feathers, as he intended to move the creature's remains into a storage compartment and they were an impediment due to their placement and size. As Spock plucked out and then lifted a feather, intending on moving it to the counter nearby so he could pull out the others, the tip accidentally brushed lightly over the side of McCoy's neck.

Spock turned his head with a raised eyebrow when his companion emitted a startled squeak and jerked away.

"Doctor? Are you well?"

McCoy giggled lightly, and removed his glove so he could rub the side of his neck. "Sorry, it's just-be careful with that thing, I'm ticklish."

The other eyebrow rose to accompany its companion into Spock's hairline. "Ticklish?"

"Yeah, it's a thing where certain areas of your body are more sensitive to stimulation than others due to clusters of nerve fibres. Didn't your mother ever-" He stopped. "Sorry, nearly forgot who I was talking to for a moment."

The Vulcan felt some curiosity about what he was going to say, but decided it was less important than his current curiosity about this 'ticklishness.' Experimentally, he brandished the feather and wiggled it at McCoy's neck again.

The doctor squeaked again, and rushed out of reach to the other side of the examination table.

"Spock!"

Spock followed him, still holding the feather. "I am merely trying to understand this unusual reaction you are having. You laugh when you are tickled, which humans generally use as a method of expressing pleasure, yet you keep attempting to escape the sensation. Why?"

McCoy tried to flatten himself against the wall, still giggling. "It's an instinctive reaction-eek, no!"

The Vulcan half of him was wondering if the course of action he was taking wasn't a little, well, he hated to use the word but there seemed to be no better one available, silly. It was certainly illogical to postpone storage of the corpse and bringing his findings to the captain simply to satisfy an idle curiosity, especially one that seemed to create such conflicting reactions in the subject of his study. The human half sidled up to it and reminded it that there is some logic involved in full examination of important new discoveries, such as a condition that might one day prove dangerous to the doctor, if his incapacitation in the face of being tickled was anything to go by. Of course, the fact that he was practically chasing McCoy around and around the table with a feather was not the most dignified way of going about it, but even a Vulcan could not have everything.

"Are there any other areas which possess this level of sensitivity?" he asked as he tried once again to corner his test subject.

"No way am I telling you!" McCoy barked. He attempted to dodge past Spock, but found his way blocked.

An affronted eyebrow was raised, just in time for them to be interrupted by a voice demanding, "Do I want to know what's going on here?"

As you may have guessed, the voice belonged to the captain, who was standing in the sick bay's doorway, hands on hips, a very confused expression on his face.

Spock ceased his pursuit, slowly lowering the feather and definitely not feeling very self-conscious all of a sudden. "I am attempting to conduct an experiment on the sensitivity of Doctor McCoy's nerve clusters."

"He's tickling me!" McCoy protested indignantly.

Kirk's jaw dropped, before he finally managed to choke out, "Tickling?"

Spock nodded. "The results are somewhat...inconclusive, because the doctor is being uncooperative. Are you by chance ticklish, Capt-?"

He was answered by the sound of clattering footsteps racing down the hall.

One eyebrow raised, Spock turned back around-and found that the doctor had taken advantage of his distraction and made his own escape into his office, hurriedly enacting the override lock.

It was a term he found himself using a lot with these humans, but…

"Fascinating."

Is this crack/OOC?

Possibly.

Am I sorry?

Not even a little.