I love thinking about how Papyrus and Sans work (for lack of a better term) and figured that Frisk would probably be curious too. Also I wanted to write fluff so have a one-shot.

Toriel hummed lightly as she worked in the brightly lit kitchen, Papyrus and Frisk sitting at the small table eating their freshly prepared lunch. Well, Frisk was eating. Papyrus was doing more chatting than any actual eating. Unbothered, the child nodded along, waiting patiently when every so often he paused to take an enormous bite of his sandwich before resuming his monologue.

It was a peaceful Saturday in the Overworld and Toriel smiled as she looked out the window to see the bright, cloudless, blue sky. She never tired of seeing the sky and all the things it held, especially the sun, which she now noted was midway through its climb.

Drying the dish she had just been washing, the goat monster gently put it back in the cabinet and turned around to observe the progress Frisk and Papyrus were making on their lunches. The former had finished, giggling disbelievingly as the latter shoved the rest of his meal into his mouth. Toriel grimaced at the sight.

Sans hadn't come out of his room yet, she noticed, despite it being past noon. While he may not get as much rest as he needed most nights, and was therefore tired quite often, Sans was never one to sleep in for so long. Papyrus always complained about his brother sleeping too much but Toriel, a fairly early riser, had yet to wake up without Sans already being out of bed. The shorter skeleton brother did have a habit of taking afternoon naps however, so maybe that was how he always managed to be the first one awake every morning.

"Frisk dear," Toriel said thoughtfully as Papyrus scooped up their lunch dishes and began washing them in the sink, "why don't you go wake up Sans? He probably shouldn't sleep all day."

"YES! GO ROUSE THAT LAZYBONES! HE CAN'T SLUMBER THOUGH A PERFECTLY FINE DAY SUCH AS THIS!" Papyrus called over his shoulder. Frisk gave a determined nod and trotted off towards the stairs, Toriel smiling fondly at the child's serious expression as they went to complete their task.

Bouncing up the stairs, socked feet making soft pap pap pap-ing noises on the carpeted floor, Frisk went straight for Sans's bedroom. They paused a moment at the door, knocking politely and waiting for a response. When there was none, they gave a mental shrug- figuring that Sans must still be sleeping- and turned the nob, calling out a soft "Sans?" as they peeked into his room.

As always, Sans's room was a mess. There were piles of clothes strewn about the floor and stacks of books cluttered around his paper-covered desk. Sans himself was still in bed, curled up in a lump under his sheets and pillows.

Frisk padded over to stand at the side of his bed, watching the motionless lump under the sheets. "Sans?" They called curiously, but received no reaction. A stubborn pout twisted Frisk's lips and they huffed. Climbing onto the bed they started digging through the covers, trying to find the skeleton underneath.

"Sans! Time to wake up!" They tried again, this time hearing a sleepy grumble in response. Frisk watched in annoyance as Sans shifted a little only to settle back under the blankets, his form now rising and falling with his breathing that Frisk hadn't noticed was absent before. Their small hands patted around again, this time trying to determine where in the pile Sans was positioned by touch.

"Saaaaans!" Frisk wailed playfully, finally finding his skull and giving it a friendly rap with their knuckles, "It's past noon, get up!" There was a hefty sigh and then Sans was flipping the sheets away from his face to level a half-hearted glare at Frisk, though the amused grin took away any heat from the expression.

"Alright kid I'm up, I'm up," he said tiredly, voice gravely, and sat up. Frisk cocked their to the side sharply as Sans slouched forward and rubbed his eyes with the heels of his hands, the sheets pooling around his waist. He wasn't wearing a shirt, Frisk could see every bone of his torso, something they'd never seen before. Papyrus had no problem being shirtless, and it's not like bones were anything new or odd to Frisk, it was simply that they had never seen Sans without a shirt or jacket.

Sans dropped his hands into his lap heavily, turning to look at Frisk only to find the child giving him an odd stare. The skeleton's sleepy smile drooped a little, clearly confused.

"You're not wearing a shirt," they said as if they'd read his mind, reaching a small hand out to poke at the head of his humerus where it connected to the other bones that made up his shoulder. Sans snorted, lop-sided grin back on his face once more.

"Oh, yeah. Sometimes I don't sleep with a shirt on," he said flippantly, bony fingers tapping on his sternum with soft clicks. "Why're ya staring though, kid?" He questioned, observing Frisk's look of concentration as they prodded at his elbow joint, picking up his arm to move and bend it, watching the bones glide across one another smoothly.

"How do you work?" They said, scooting forward a little more and moving their hands to inspect his scapula. "Uh," Frisk suddenly paused, "is this okay?" They muttered nervously, unsure if their exploration was considered inappropriate to their skeleton friend.

Sans laughed loudly, the sound instantly putting Frisk's nerves at ease, "Nah kid it's fine. Kinda feel like a massage to be honest." His expression turned thoughtful, "Er, at least what I think a massage would feel like? I don't have any squishy muscles like you after all." Sans finished with a shrug, mindful of Frisk's fingers so as not to let them get pinched between any bones with the motion.

"Oh," Frisk replied with relief, "okay." There was a moment of companionable silence as Frisk continued their poking and prodding, Sans looking on in amusement, before Frisk asked again, "How do you stay together?"

Another shrug, though this time it was accompanied by an almost smug looking grin, "Dunno kid. Magic." Frisk huffed at the unsatisfactory answer, cheeks puffing up and brow furrowing to show Sans their displeasure. The skeleton merely gave a guffaw that he only half tried to pass as a cough.

"Really Frisk, I'm a monster. Monsters are made of magic, that's all I got for ya," he insisted with a snicker. Frisk's stubborn grimace threatened to morph into a smile at seeing their friend laugh, so they just shook their head in exasperation and went back to examining Sans's bones.

They moved from his shoulder to his hands, their small fingers plucking at Sans's metacarpals and phalanges in fascination. There were no muscles or tissue to keep the metacarpals close together, allowing Sans's fingers to spread farther apart than Frisk's ever could. Sans chuckled at Frisk's look of wonderment as they toyed with his hands, placing their palm against where his would be and watching his distal phalanges curl over their fingertips with a giggle.

Soon enough Frisk was moving on, fingers traveling into the space between his radius and ulna, feeling the bumps and ridges of his spine, and finally hovering uncertainly over his rib cage. Sans, soothed by the soft touches and still drowsy from sleep, found himself hunched with only a hand propping up his head and heavy eyes closed tiredly, letting Frisk continue their examination and only speaking to answer the occasional question.

He snapped upright though, a shock traveling down his spine and jolting him from his half-doze, when Frisk experimentally stuck their hand through the space between two of his ribs. The child jumped at his sudden reaction and was quick to snatch their hand away, worried they'd done something wrong.

"Careful kid that tickles!" Sans said with a slight wheeze, not realizing his mistake before he noticed Frisk's expression turn from one of concern to mischief. "Wait n-ah!" He choked, laughs bursting from him in rattling motions as Frisk managed to worm their fingertips into the ticklish area again, a huge smile growing on their face.

"FRISK WHAT'S-" Papyrus stopped mid-way into Sans's room, disgruntled frown falling from his face as he watched Sans cackling, tears streaming from his eye-sockets, under Frisk's merciless assault. Sans managed to pry open an eye, hiccupping and wheezing loudly in nearly soundless laughter.

"Pap-!" he cried desperately, "Pap! Help!" Papyrus gave his brother a mock salute and strode quickly over to Frisk, grabbing the child under their arms and lifting them away from Sans.

"I WILL SAVE YOU BROTHER!" He yelled, holding Frisk at arms-length as they squirmed and laughed happily, kicking their feet in an attempt to free themselves from the lanky skeleton's firm grasp. Sans took a few deep- though unnecessary- breathes, standing up on his bed to glare playfully at the struggling child.

"You're lucky I don't actually need to breathe kid or ya' woulda' killed me!" He cried dramatically, trying to hide his grin while wiping the tears from his face. Frisk only smiled and giggled again, Papyrus bringing them in close to his chest in a legitimate hold, supporting them with one arm as he used the other to poke his fingers at Frisk's sides.

"HOW DOES IT FEEL TINY HUMAN?" Papyrus cackled as Frisk squirmed in his arms, hiccupping laughter interrupting their cries for mercy. "WHAT EVEN IS ALL THIS," he teased, jabbing at their tummy, "WHAT IS ALL THIS SQUISHY STUFF? WHAT DO YOU NEED IT FOR?"

Sans smiled fondly as he watched his brother play with the kid, though after a minute he gently reached over and grabbed his brother's hands, stilling them from their torture.

"Alright Pap I think they've had enough, the kid needs to breathe," he chuckled, both skeletons looking on as Frisk settled in Papyrus's arms, gulping down lungful's of air with a massive grin on their face, cheeks flushed red from all their laughing.

"AH YES, QUITE RIGHT BROTHER! HOW COULD I HAVE FORGETTEN?" Papyrus gasped in mock despair, leaning back and throwing a gloved hand over his eyes. The gesture elicited more giggles from Frisk, who reached up to tug the appendage away from his face. Papyrus joined in their merriment with a laugh of his own, the moment interrupted by Toriel calling them from downstairs.

Sans smiled lazily and made a shooing motion with his hands, "Okay you two, out out." He ushered them towards the open door, "I'm gonna get dressed and I'll meet you two boneheads downstairs in a minute." Frisk was set on Papyrus's shoulders, the tall skeleton ducking under the doorway as he made his way downstairs, chatting with Frisk once again. Sans sighed with a smile, shaking his head and closing the door behind them.

"Well that's one way to wake up." he muttered softly to himself, unable to keep his grin widening as the fondness for his family settled like a glowing warmth in his chest.