Hello ! Here's a Christmas OS I wrote for a Secret Santa event I participated in ! I reallyyyy love SenGen but it was my first time writing about them, and writing those two jerks was as hard as I thought it would be xD
I hope you'll like it :D

Enjoy !

The icy winter air was rushing through the door of Senku's laboratory. A few gusts of wind blew the curtain from time to time, and Gen's fur-lined coat. The shivering shivers would slide down his skin each time, as he struggled to stay concentrated in the scientist's monologue that reached his ears.

The only advantage he found in this situation was that the clear entrance gave him a view of the landscape surrounding the Kingdom of Science. The snow had already begun to fall a few days earlier, so that the vegetation was now covered in its faithful white coat, heralding a winter that would probably be harsher than the previous one. The flakes swirled in the breeze, landing on the already fluffy surface of the ground, in the branches of trees and on the roofs of houses. Cotton life that embraced him with nostalgia.

It was while contemplating this view that Gen realized how ephemeral and eternal snow was. Three thousand seven hundred years later, it kept returning, every year, to connect him to what the modern world was like, to the vision of a white city that he had been able to observe for nineteen years. According to the calendar Senku now had – it seemed that writing down the days was more convenient than counting and memorizing them – and which indicated December 23, it had been ahead of the previous year. Once again, they would spend a Christmas under the snow, even though the inhabitants of Ishigami village did not grasp the subtlety of such a celebration.

And even though the only one who could grasp it didn't care about it.

"Oi, mentalist. You're listening or what?"

If Senku hadn't raised his head, Gen nevertheless easily guessed that he'd given him the eye, probably because of his silence and lack of concentration.

"Not at all, Senku-chan," he answered honestly, in a singing voice and with a broad smile on his face. "What were you saying?"

"I was asking you to give me the copper sulfate," the scientist repeated, pointing with his index finger at the round vial filled with a blue liquid, next to Gen.

He considered the container for a few seconds before grabbing it. The fluid mutated under the virulence of his gesture, leaving him unable to turn his dark gaze away from it. The transparent bottle, made of glass, not especially perfect, round like this and filled with the azure substance, made him think of a Christmas tree decoration ball. The timid daylight, stifled by the cloudiness of the sky, in this fall of flakes, brought a soft color to the blue of the copper sulfate, so much so that this mixture alone was enough to inspire the mentalist.

"Senku-chan," he hummed, turning to him, without handing him the bottle he so much wanted. "This vial..."

"Kukuku, you also are fascinated by the fact that the dehydrated white powder of copper sulfate turns blue when it comes into contact with water?" Senku asked, his crimson eyes illuminated by enthusiasm.

Frozen silence. Gen almost felt as if the winter breeze had just slipped under his clothes, so cold had these words been.

"Uh... No, not really," he replied, before pulling himself together. "I thought it looked like a tree decoration. Christmas is tomorrow," he continued, pointing to the calendar in the entrance of the laboratory. "We could do something with it, don't you think so, Senku-chan?"

"No way, give me back my vial before you drop it, I need it."

Without bothering to do so, Gen turned around and looked at the other containers in the lab. In chemical reactions that were completely beyond his comprehension – he just used to trust Senku, without ever understanding what he was doing – different colours, more or less bright, decorated the table, in vials that were more or less identical in their roundness.

A sigh escaped the mentalist.

"The others are all preparing the village for tomorrow, why are we stuck in there?" Gen whimpered.

"Nothing forces you to stay," Senku replied, without looking up at him, grabbing the copper sulfate from his hands. "You can just go and help them, they'll need someone from the modern era, kukuku."

Indeed, despite their lack of knowledge of what Christmas was all about, the inhabitants of the village felt invested in its preparation. The huge illuminated Christmas tree from the previous year had aroused their curiosity and, after authentic stories from the modern world people about what these celebrations were, everyone agreed. This year they would celebrate Christmas. As they could, anyway.

Hesitation seemed to run through the mentalist then, under his reflection and in order to fight against the cold that crept into the laboratory, he pushed his wool cap even further down on his head. On the one hand, the idea of taking part in the preparations for the festivities was more than tempting, much more than doing he-knows-what in this icy laboratory, standing still. But on the other hand, the prospect of this one-on-one with Senku was equally appealing. After all, in the midst of all the people in the village, they rarely had the opportunity to be alone like this - not enough to his liking, anyway. And although it seemed to be the least of the scientist's concerns, Gen was satisfied with the situation.

"Don't talk nonsense, Senku-chan, I know you want my presence," he said. "That's why you dragged me here~"

"But you're useless to me now, mentalist, so make yourself useful."

He didn't bother to deny it or find an excuse. He wouldn't admit it, but it was true that he could very well have asked Chrome, which would have been a much better choice, rationally speaking. But he didn't. For some time now, it was Gen whom he found himself calling on when he needed someone.

An irrational feeling. An aberrant and illogical one, against which he had finally stopped struggling. The more he remained focused on the future of the Kingdom of Science, the better off he would be, after all.

"Oi, what are you doing?"

A grimace of exasperation arose on the face of the youngest, in front of the lack of response. His pupils shrank in surprise, allowing his scarlet irises to stand out even more. His eyes followed Gen's hasty movements as he grabbed various objects here and there and gathered them all together on the table that occupied the main space of the room. Had he just had a scientific enlightenment, as he often did?

It was possible, after all. Gen may have been a frivolous man when he had met him more than a year ago, but the mere fact that he had joined his side by leaving Tsukasa's proved that, despite his carefree and superficial nature, he was far from being a fool. It was likely that he had come to understand the interests and fascination of science.

A few metal rails, which incidentally served as a lab support to hold everything and nothing, were assembled in what looked like a pyramid. As the seconds rolled by, hopes of seeing Gen doing something useful and meaningful diminished. He was quickly grabbing the vials, almost spilling them, even mixing them up with every gesture, yet his lips gave off a hiss of bliss, amusement.

"What are you doing, mentalist?" Senku asked again, as he started to lose patience.

"Ta-da!" Gen said, retreating to allow him to observe his masterpiece, a broad smile on his lips.

Attached to the base that usually held some of the laboratory's glassware, especially the round-bottomed balloons, a vertical iron rod held others, perpendicular and shorter as they rose. The vials of different colours that previously occupied the table had been attached by what served as clamps to this improvised vial stand, to Senku's great incomprehension.

"What is it?" he asked in a circumspect tone, reflecting his perplexity.

"Your laboratory was empty, Senku-chan. I thought we could try to decorate it for Christmas~"

"That doesn't tell me what it's supposed to be," he replied, pointing to the improvised pyramid.

"Well, a Christmas tree, that's obvious, though. I could have added my scarf as a garland, but I might be cold without it!"

"The only thing that's obvious is that you got it all mixed up. Next time I need the copper sulphate I'll get it myself," the scientist sighed, scratching his ear, as he usually did.

By the time they'd been around each other, now, Gen had learned to read through his words. Nothing more normal for a mentalist, after all, but it was because of this that he could perceive the faint glow on the surface of his scarlet eyes much more easily than anyone else. Even though Senku constantly claimed that science was all that interested him, he knew better than anyone else how much, deep down, at the edge of unconsciousness, he paid attention to relationships and things in their entirety.

The simple fact that he had feigned ignorance the year before by lighting the huge tree on Christmas Eve proved it. Then Gen only had to observe the surprise that stretched his features almost imperceptibly and created slight folds in his forehead, his lips threatening to split into a thin smile, the raw breath leaving his nostrils, through a little laugh that he couldn't hold back. The wick that barred his face rose very faintly, and judging by the furtive wink he was subjected to, it was easy to guess that it had probably tickled him.

An icy gust of wind, more virulent than the others, entered the laboratory to slide over his skin and connect Gen to reality. A few flakes of ice slipped into the lab and slowly fell to the floor, the table and the improvised tree, only to die immediately. Ephemeral and eternal snow.

No more noise or shouting reached them, a sign that everyone had probably left to go and decorate the giant tree. It was at this observation, now again aware of the world around him, that the mentalist swallowed well despite himself. His clenched fists, inside his joined sleeves tightened, as did his jaw, as he looked at the face facing him. Close. Too close. Gen couldn't tell when their bodies had come so close, too busy paying attention to the details he could read in Senku's most innocuous gestures.

The eye contact they maintained, each as unwilling as the other to break it, had the merit of speaking for them. Under normal circumstances, Gen would probably have taken the trouble to look around to make sure they were alone. But the sounds of winter, devoid of the slightest human voice, made him lose all patience. As did Senku's gaze as he clung to his own. Heavy with meaning and charged with so many emotions that he let his face crack with a simple smile.

"We can still consider it's mistletoe, and not a glass tree," he let go, pointing to his pyramid of vials with a thumb sign.

This characteristic smile again drew the scientist's lips, even more enticing than anything Gen could have vainly hoped to fight against.

"It doesn't make any sense, mentalist, we're not even underneath it."

If he didn't pay attention to every detail as he did, Gen would probably have missed the faint redness that had just appeared on his cheeks. As subtle as it was, it was more convincing than he could have expected. Then, with unparalleled gentleness, his fingers grasped the fur lining of Senku's coat to get him even closer. And before he knew it, every space between their bodies was reduced to nothing, starting with their lips. Greedy and shy lips, eager and hesitant; in the sweetest and most tender intimacy.

As ephemeral and eternal as snow.

I hope you liked it !
Please don't hesitate to leave a review, it's really important to me :3