There wasn't too much that Sans understood about humans, biologically or socially, if he was being honest. Learning that they apparently all had skeletons in side them was definitely on top of the list of weird things he and Papyrus had discovered, but he often found himself puzzled by smaller things too. Things like how some humans regularly put toxic or harmful substances into their bodies willingly.
At first, smoking was a beyond confusing topic to Sans. He'd first been introduced to the activity when some of his monster-friendly coworkers had invited him to hang out with them on their smoke break. Sans hadn't really known what a "smoke break" was but if it meant he got to chat and not work then he was all for it, so he'd accepted and ambled after the group as they made their way outside.
Turns out "smoke break" meant using cigarettes, which Sans was only vaguely familiar with, but the human version of cigarettes seemed to be wildly different than the monster version. For one, human cigarettes stank to high heaven. They were also, Sans discovered after asking, full of stuff that made them addicting and could be rather unhealthy, though the humans shrugged that off.
Sans had absolutely no clue why people would inhale shit that was bad for their internal organs- something that most living, non-monster entities seemed to have- another odd thing Sans had learned- and only warily agreed to try one when it was offered to him.
"Can you even smoke?" One woman, Kate, asked as she handed Sans her cigarette, "I mean, you don't have lungs right?" Sans merely shrugged because no, he didn't have lungs and didn't technically need to breath, and yet he did it anyway so he could probably smoke.
"Only one way to find out," He replied, bringing the roll of paper to his teeth, mimicking what he'd seen the humans do just a moment before, and taking a deep breath.
At first they laughed, Sans making a face at the unpleasant flavor and smell, but the laughter quieted when smoke started to spill out from between the skeleton's teeth, now tinted an electric blue. The blue smoke coiled through the air around his head for a moment before the breeze took it away, leaving the humans looking confused and intrigued.
"Dude how'd you do that?" Tyler asked, grinning eagerly like he wanted to learn. Sans shrugged again, content to leave it at that, but when met with the disappointed faces of his coworkers decided to give them his theory.
"Monsters aren't like humans at all," He began, eyeing the smoldering cigarette held between two of his bony fingers, "The food and stuff we put into our bodies never has a need to come out because our magic absorbs it all. Unlike you guys, who's bodies only use parts of what you consume before throwing the rest out," he paused, making sure the human crowd around him understood before continuing, "So I guess, and this is only a theory, that since my magic usually consumes what I eat or whatever, it did the same with the smoke before I blew it back out- which is why the smoke was blue."
There were hums of wonder as his friends processed this information, contemplative- or in Tyler's case, disappointed- expressions on their faces. Not long after that their break time was over and everyone returned to work, Sans temporarily pushing the subject from his mind for later.
It was three months later when Frisk caught him, the child having somehow known he was on the roof late at night- though Sans really shouldn't have been surprised, Frisk had a knack for knowing when he had nightmares just like how he always seemed to know when they suffered theirs.
The carefully clambered out of Sans's open window, blanket wrapped around their small shoulders like a fluffy cape as they settled beside the skeleton already sitting there, gazing up at the stars with luminous eyes.
"Heya kiddo," He greeted without looking away from the sky. They didn't say anything, but Sans could feel their eyes trained on him, drawing his gaze to theirs curiously. Frisk pointed at his hand wordlessly, looking to him in question.
"Oh," Sans said somewhat sheepishly, also looking at the vape pen held lazily in his right hand, "It's a uh vape pen," he explained, "people usually use it to quit smoking because they can put different amounts of stuff in them but I like it for the tricks." He finished with a shrug, casting a sidelong glance at the wary child beside him.
"Tricks?" Frisk inquired with head cocked to the side, their curiosity piqued, and drew closer to Sans's side.
"Yeah, I don't really put much in this thing," he wiggled the pen a little and brought it closer to Frisk so they could see, "I just like watching the vapor. Plus it's way better for you and everyone around you than actual cigarettes." The child leaned a little closer to get a better look at the pen, making no move to touch it, and looked at Sans once more.
"Can you show me?" They asked with muted excitement, intrigued by Sans's words. The skeleton chuckled with a small grin, bringing the pen to hover in front of his teeth.
"Sure kid," Sans said before taking a deep pull from the pen. Frisk watched in fascination as Sans tipped his head back and blew a heavy cloud of vapor from his nasal cavity, like a dragon spouting smoke before it breathed fire. The vapor nearly glowed it was such a bright blue, and Frisk easily recognized the color of Sans's magic. Both of them watched as the vapor drifted away like a lone blue storm cloud in the wind.
"More?" Frisk asked excitedly, bouncing a little and tugging on Sans's free arm. The skeleton laughed again and took another breath of vapor, this time letting the steam seep from his eye-sockets, the already color-tinted cloud becoming even more haunting as it caught the dim light of the skeleton's eyes.
Frisk giggled, swiping a hand through the cloud and watching it disperse, Sans already releasing a round of smokey rings from his mouth. Frisk's laughter increased and Sans smiled, happy to see the kid enjoying themselves.
"How do you do that?" Frisk asked in awe, eyes following the rings as they drifted away and broke apart.
"The tricks?" Sans asked, resting the hand holding the pen on his knee, leaning back on his other hand. Frisk shook their head, short hair tossing from side to side.
"No, how do you keep it in?" They clarified, poking Sans's side through his thick jacket, referring to the skeleton's lack of lungs "How does it stay?" Sans thought for a moment, gaze returning to the stars as he tried to think of a decent and understandable answer. He hummed after a moment and turned to look at the child beside him.
"Yanno kid I'm not certain, I never really thought about it. Maybe that's why. Because I didn't think about it," He shrugged with a smile, "Wanna test it?" Frisk bounced a little in excitement, clapping their hands and giving Sans a smile of their own.
The skeleton shrugged out of his jacket, draping it around Frisk's shoulders, and took off the plain T-shirt he wore underneath. He saw Frisk wiggle their fingers, a small mischievous smile on their face, and knew they were recalling how ticklish he was. Their hands stayed in their lap however, when Sans gave them a playful warning glare before once again taking a pull from the pen, noting that it was running on empty.
Frisk watched in fascination as the vapor was pulled into Sans's chest cavity, the cloud swirling inside his rib cage reacting to his magic and becoming more and more blue the longer Sans held it there. It looked like a radioactive storm cloud, confined to a bone prison, coiling around itself and casting an eerily dim light on the skeleton's ribs.
They let out a surprised yelp when the vapor suddenly rushed from the gaps between Sans's ribs, the skeleton making a whooshing sound like he was forcefully exhaling. Skeleton and child watched as the last of the vapor drifted away, lost in their own thoughts for a few minutes afterward.
"Alright kiddo, time to go back to bed," Sans finally spoke up, standing carefully and offering a hand to Frisk. They took it without hesitation, nodding with a yawn, and Sans teleported them back inside to tuck them in.
"G'night Sans," Frisk mumbled while Sans pulled their comforter up to their shoulders, dropping a gentle "kiss" to their hair.
"Night Frisk," He replied quietly, watching for just a moment as the child closed their eyes with a contented sigh before leaving, closing the door silently and slipping across the hall to his own bed, feeling much lighter and ready for sleep than when he'd originally woke up.