"so what was it you needed help with again?" Sans stood in the middle of a large white room, the walls were lined with different kinds of electronics and the tables were filled with different mechanical components, Frisk walked around the room examining everything curiously.

"I need you to test my new invention." She gestured to a machine in a corner, it was booted up and making a weird whirring sound, it was just a big tube like chamber with a series of computer interfaces attached to it.

Sans nodded, "and what does it do?"

She walked over to him, carrying a tray of various medical objects. "So far it can make monsters temporarily human, ideally when I finish tweaking it, it will be able to turn humans to monsters as well."

Sans looked at his hands, "What is the purpose of that, its not like we have to hide our identities…"

She looked thoughtful for a moment, "There has been a high demand for such a procedure since we reached the surface, different humans see us and find they aren't comfortable in their own skin, that they feel they should be one of us, there are also some monsters that feel the same way…"

Sans thought about it for a moment, he guessed it would be a good thing to feel comfortable being who you feel you are, even though he couldn't understand how that felt. "okay, but why didn't you get Undyne to help?"

"I did, but she has flesh…" she pointed at him, "I need to see if it will work with someone without any."

Sans looked satisfied by that answer, "Fair enough… there's no negative side effects?"

"everything has side effects Sans…" she sighed, "but yours should be limited having to get used to having human skin and organs and…" she cleared her throat, "other things you maybe aren't used to… and you may find you're very hungry after its done, since you go from having no stomach to having a stomach, I have some fast food waiting by for when that happens."

He nodded, Frisk walked over, "are you sure about this?" concern for her friend plain in her voice.

"yeah I'll be fine Frisk, I've always wondered what I would look like with skin." He smiled reassuringly at her, she nodded.

"alright, thank you Sans." She said handing him a wrap around gown, "no metal in the machine so you have to wear this for safety."

Sans took the gown and stripped down, both Frisk and Alphys turned away, he laughed putting the gown on, "okay I'm ready."

He stepped into the machine, Alphys closed it and moved over to the control panel, Frisk stood by and watched, fidgeting nervously.

"okay everything is set up," Alphys announced a few minutes later, "I'm going to count down from three, then I'll start the program," Sans nodded, "Three…two…one!" she pressed a button and a blinding light filled the machine.

Sans felt weird, like he was growing really fast, it hurt but not enough for him to feel concerned, he expected it might, he tried to watch his feet but the light hurt his eyes, he felt a pain in his stomach, and a steady thumping suddenly in his chest, he felt warm, very warm, and when he tried to look around he felt something moving in his head, he wasn't expecting it to feel this foreign to him.

Suddenly the light died down and the door opened, he stepped out, flinching at how cold the tiles felt on his bare newly skinned feet.

He heard Frisk gasp, she observed his change, frozen in place, he was slimmer then she thought he would be and very pale, he had huge bright blue eyes and fluffy blonde hair on his head that contrasted with his dark eyebrows. She walked stiffly over to him, unable to believe what was in front of her.

He laughed, pausing quickly, startled by the vibrating in his throat, "you look like you've seen a ghost."

She remained silent as she came to a halt directly in front of him, exactly eye to eye with him, she observed every little detail of his eyes, they were astonishingly beautiful. Sans looked nervous, she reached her hands up and began to squish his face.

"oh my god…" she stated, shocked, "oh my god you are precious…"

Sans laughed, and Alphys handed him a mirror, he looked into it and jumped, moving his hand to his face.

His behaviour continued like that as he sat and scarfed down two whole meals from a nearby restaurant, went over how he felt and some basics of human anatomy with Alphys, and had a quick physical, she brought a mirror into the examining room for him to get used to his new body while he got dressed.

The addition of his permanent member shocked him and made him feel weird, he could always use magic to conjure one but never really felt the need to, having one he couldn't control seemed uncomfortable, he didn't understand the concept at all, he always thought magic made more sense.

After getting dressed and exiting the room he heard Frisk talking to Alphys in hushed tones.

"so he does?" Frisk whispered, neither of them had noticed him yet.

"why do you want to know?" Alphys replied, nudging her with her elbow.

"I'm just curious!" Frisk whispered defensively.

Sans cleared his throat, the act startled him almost as much as the two girls, they turned to face him, Frisks face was beet red.

"what are you two talking about?"

Alphys struggled for an answer for a moment, "just… uh… discussing any complications you might have!"

He looked at her skeptically but dropped it, "hey Frisk we better get going, Paps is going to flip when I get home."

When they got back to Sans' house, Papyrus flipped out just as Sans said he would.

"hello Frisk! If your looking for Sans he isn't here right now…" he looked to Sans, "Oh look, you brought a friend! Who is this?"

Frisk snickered as Sans spoke, "hey bro, don't you recognize me?"

Papyrus stood frozen for a moment, the words and voice seemed to make sense to him but he couldn't seem to fit it to that face. "Sans?" he spoke cautiously.

"yeah Pap, I thought you would recognize me even if I did have skin." He said hands in his pockets.

Papyrus immediately panicked, "Who did this to you?! Is there any way to change back?! Why would you want to do this?!"

Frisk stepped forward catching Papyrus by the shoulders forcing him to calm down a bit, "Papyrus it's just one of Alphys' experiments… its temporary, Sans should be back to normal in a few days."

Papyrus nodded, she let him go and he walked over to Sans, he stared at him intensely. Sans felt nervous, until he felt his feet lift off the ground as Papyrus picked him up in a boney hug.

"Uh…Papyrus?"

"you're adorable!" he exclaimed, Sans felt his new blood rushing to his face.

He wiggled out of his grip, Frisk was giggling, "I'm not a child Pap…" he pouted.

Frisk stayed at their house for dinner, they had spaghetti and pie Toriel had sent over the night prior for dessert, Frisk and Sans played some video games until Frisk had to leave.

"Good night Frisk, see you tomorrow?"

She nodded, "sure!" she stepped forward and gave him a hug, a normal gesture, but this time he could feel how warm she was… and how… curvy. He felt blood rushing to another place this time as she exited the house and began her short walk home.

Sans ran up the stairs, dialing Alphys, she picked up on the second ring, "hey Sans, how's the flesh treating you?"

"uh… fine, I think…" he paused, embarrassed, "do you think I have feelings for Frisk."

"Sans everyone thinks that…" she stated matter-of-factly.

"okay…" he didn't think it was that obvious… "well is it normal for me to feel really… hot?"

"that comes with having flesh, yes." She said, he could hear papers shuffling in the background.

"okay good… is it normal for my… thing… to get all hard like this when I don't want it to?" at that question he heard something shatter and the phone clatter to the ground.

She picked it up hastily, "oh…uh… I-I guess I forgot to go over that with you huh?" she said nervously.

"I'm listening." he stated, slightly annoyed.

"well…um…let's see… when human m-males feel attracted to someone, close physical contact may cause their… p-penis… to become erect… its called an erection, its an evolutionary trait, it makes it easier to… reproduce…" she seemed very uncomfortable.

"well I know that! I just thought I would have more control over it..." He was embarrassed, "how do I get it to stop?"

She hesitated, "give it some time? If it's uncomfortable you should maybe… t-take care of it… yourself…" she seemed perturbed at the thought.

"well how am I supposed to do that?!" his throbbing manhood was getting increasingly painful to have.

Alphys became frustrated, "I don't know! In case you haven't realized I've never touched one! I'm into g-girls…"

Sans felt bad, he was obviously making her very uncomfortable, "I'm sorry Alph, it just startled me that's all."

She sighed, "that's alright Sans, I shouldn't have yelled at you like that, you're just confused…" she paused for a second, "maybe ask the internet, I'm sure you'll find something there…"

"okay Alphys, thanks."

"no problem."

A couple hours later, all he could find were videos that kind of grossed him out, but also just made him more aroused, so he gave up, resolving to distract himself with mindless videos he brought up his favorite website. But he couldn't stop thinking about Frisk, and his problem wouldn't seem to go away as long as that kept happening.

A few minutes later there was a tap at his window, he walked over to it seeing Frisk perched on a branch just outside the window. Oh god… not right now… he thought opening the window and letting her in.

"what are you doing out at this time?" he said, trying to keep her attention off his pants, he was lucky the room was dark.

"couldn't sleep, came to see you…" her face was flushed.

He went to respond but stopped when he saw her looking deeply into his eyes, he froze as she stepped forward putting a finger on his lips, "hmm," she hummed thoughtfully, "your lips are very soft…" her eyes were hazy.

"uh Frisk?" she pulled her hand away, he noticed she wasn't wearing much, a black long-sleeved button up blouse and a pair of really short jeaned shorts, he could barely see them under her top, his eyes scanned her legs, they looked so soft, he wanted so badly to touch them… to touch what was between them, he shook his head trying to clear those thoughts from his mind. When he looked at her she was staring at him observantly.

He cleared his throat, "So what's keeping you up?"

She sighed, "I don't know… but I was bored when I saw you come online I immediately headed over."

He looked confused, "I didn't sign in though…"

She rolled her eyes, "you sign in automatically when you start your computer unless you tell it not to…"

He laughed, "oh…"

She leaned against his desk, knocking a pencil onto the floor behind her, she turned around and bent to pick it up, Sans felt himself get really hot as his eyes automatically fell to her perfectly silhouetted behind. When she straightened up and set the pencil back on the table she closely examined his face.

"something wrong?"

He tried to keep his cool, "N-no, nothings wrong…" he failed.

She laughed, "seems like there is." She crossed the room, moving passed him to sit on the edge of his bed, he noticed she smelled like sweet flowers.

He took a seat beside her, "I've had some issues getting used to having skin…" he said, trying to get off the topic as fast as possible

She took his hand, "I guess it would take some getting used to for a skeleton, but it's only temporary, you shouldn't work so hard when you won't have to deal with it for long."

He could tell she was trying to make him feel better, but touching him only made his temperature rise, she clasped his hand in both hands, "Are you really okay? You feel very… clammy…"

He heard the term before but forgot what it meant, he looked at her confused, she explained, "well, your hands are cold, but sweaty… it could mean your sick…"

He laughed, "I'm not sick…"

"oh, then you know what's wrong?" she quizzed.

He realized there was no avoiding it, he felt the thumping in his chest intensify as he began to explain, "Frisk I have something to confess…" she looked into his eyes, he looked to his hands, "Frisk, I…" he paused, he really didn't want to change anything between them, "I've had these… feelings… for you. For awhile now, I didn't really know what to do with them." He looked at her, her eyes were cast aside, he felt like he was making a big mistake, but continued. "and since I've gotten this body… those feelings have become more…physical."

When he looked at her this time, her face was red, and her hands were folded on her lap, she said nothing.

"I can't stop thinking about you… I can't stop wanting to touch you…" words started to spill out of his mouth, "I can't stop feeling the need to have you here, feeling the need to taste you…" he stopped himself at the last sentence, realizing how lewd that sounded.

She still wouldn't look at him, or speak, he could feel her distancing herself from him, she began to fidget. "Frisk…I... I'm sorry… I don't want you to hate me! I just—"

He tried to continue but he was cut off when her lips caught his, hesitantly at first but when he didn't pull back she snaked her arms around him. He felt hot all over, His arms shot around her, holding her tightly, she laced her fingers through his soft hair as she deepened the kiss, her tongue entered his mouth, winding around his, she pulled him down on top of her, moaning seductively… he lost control, his hand grabbed her hair, harshly pulling her head back, she gasped but he ignored her, trailing kisses down her neck, she whimpered, he nipped her a little, she grew louder, then he sucked harshly on the skin he just bit, when he pulled away there was a bright red spot left behind.

She looked deliciously disheveled, her hair was strewn about her shoulders, her face was a deep shade of red and her eyes were half-lidded. His lips found hers again, he wrapped her up in his embrace, they kissed eagerly like this for a few moments before Frisk started to lift his shirt, he pulled away from her, grabbing her wrists and pinning them above her head. She whined at the sudden stop to their passion.

As much as it pained him to stop, he had to be sure, "Do you really want to do this?"

Her eyes begged, "yes!" her voice was breathy.

He didn't let go, staring her down, "I need you to clear your mind for a minute and really think about this."

She paused, he sighed and sat up, taking his seat beside her on the bed again, she rose and placed her hand gingerly on his lap, "I've wanted this for a long time now, I just never knew how to… go about it…" he looked at her, her eyes cast aside, "but now that you're… like me, I know how to." Her eyes met his, there was no hesitation, only longing.

"have you ever… been with… anyone else?" Sans quizzed.

She shook her head, "I've never wanted to be…"

"so then I'm your first?" he felt unsure, he didn't deserve that.

She sensed his hesitation and took his hands in hers, staring into his eyes, "I wouldn't have it any other way." She smiled brightly at him.

He sighed, smiling, "Sorry I ruined the mood then…"

She laughed, getting to her knees in front of him, "easy fix." She stated, grabbing his waist band, she looked to him for confirmation, he nodded, and she freed his erection, she gasped.

"are you okay?" he asked, concerned.

"yeah its just…" she cleared her throat, swallowing hard, "Big…" she looked very intimidated.

Sans didn't know why but pride swelled within him. she wrapped her hand gently around it, he shuddered, pleasure washing over him.

"is it really sensitive?" she asked.

"very." He said rigidly, barely able to stop himself moving his hips.

She stroked it a couple times, he fought back a groan, she seemed to like that, she bit her lip. She kept stroking, slowly and deliberately, driving him mad, he bucked his hips involuntarily, she seemed to enjoy making him lose control like that.

She gauged his reaction as she leaned forward, he watched closely as she licked the tip delicately, Sans hips moved again. She looked up at him, giving him a coy smile, before wrapping her lips around his member and sucking harshly, he cried out, she sucked again, harder this time, his hips jumped again, he groaned loudly as he felt himself enter her throat, it surprised her and she went to pull back but she stopped herself, pausing for a moment.

Sans was about to ask if she was okay, until she started to bob her head, swallowing hard around him, she moaned and he could feel her voice's vibrations, he tried to keep himself tame, but as his desire intensified he coiled his fingers through her hair, thrusting up into her throat, she moaned loudly, letting him take full control, "Oh fuck!" he exclaimed, unable to control himself, her moans got louder, she started to make very lewd gagging noises, he didn't know why, but he loved it. he felt every muscle movement and every moan as he approached the edge, he called out to her, "Ah… hah… Frisk!" she moaned again, he cried out as ecstasy hit him like a brick wall, his movements slowed as he released his seed into her throat, she swallowed it all eagerly, laughing lustfully around his manhood, he found that very sexy.

He fell back on the bed, she crawled up beside him, placing her hand across his chest. "you know Sans I really love you."

He turned to her, "I love you too Frisk." He kissed her on the forehead, she giggled.

They laid like that for a few minutes. Sans felt tired, like he could go to sleep right then… but he had some unfinished business, he sat up, "you know I feel awfully selfish being the only one having fun…" he rolled over to straddle her, her eyes were wide, "Why don't I try pushing your buttons?"

She looked shocked, he removed his shirt, and leaned down to kiss her neck, she moaned hungrily, he placed a few nips here and there, her back arched seductively as her pleasure intensified, he made his way down her collar bone before coming to the first button on her top. He popped it open with ease, hearing her breath shudder, he gently traced his hand down her chest before freeing the second button, she began to get restless as he continued on to the third and fourth, he leaned back when he undid the last button, admiring his handy work, he noticed she wasn't wearing a bra, was she expecting this to happen?

Her face was flushed, she turned her head aside, avoiding his burning gaze, she felt his weight come down on either side of her.

"God, you're beautiful…" he said, lust ever present in his voice, her eyes met his, desire burned bright in his azure eyes, she felt exposed, but entirely safe.

He leaned down, gently caressing her nipple, she moaned as the delicate pink skin began to become erect, he could feel himself becoming hard again, he did the same to the other one, before taking it in his mouth and sucking hard, she moaned loudly, he bit lightly and her hips bucked. His hand snaked down her shorts over top of her panties, he rubbed gently gauging her reaction, she cried out when his fingers went over a tiny nub, he found it again and continued to lick and suck her nipples as he mercilessly caressed her clit, "Ah, Ah, Ah…" she was losing control of herself, but she wasn't done toying with him yet…She grabbed his hands and regained control of the situation, laughing wildly, throwing him to the floor, an act she thought would take more effort.

She stripped off her shorts and straddled him on the floor, his new erection gently rubbing against her through his pants, she shrugged her open shirt off, leaning over him and kissing him passionately. He could feel her breasts on his bare chest, she was so warm and soft. He could hear her humming deep in her throat as she moved to kiss and nip at his neck, He could feel himself getting harder and harder as she continued slowly, she sucked hard on his neck, marking him as her own. he needed her now… and she wasn't moving fast enough.

He grabbed her roughly, rolling her over so he was on top of her again, a lusty smile made its way to her face. His hand found its way to her panties again, this time going inside, she jumped, his fingers immediately went for her clit again and she cried out loudly, he trailed his finger down a little until he found her entrance, gently sliding his finger inside, he watched her as her eyes rolled back and she moaned deeply, he moved his finger in and out before inserting another one, he felt her tighten up around them, she began to pant as he fingered her, he used his thumb to massage her clit again. Before long she was clinging to him, moaning loudly and panting frantically.

He removed his fingers, they came away damp and sticky, she looked confused, he gave her a seductive smile as he licked his fingers, she bit her lip again, he sucked his fingers clean before gently lifting her hips off the floor, he leaned down and Frisk covered her face with her hands, he chuckled before giving her womanhood a tentative lick, she tasted sweet, this elicited a delicious moan from deep in her throat, he moved to her entrance, plunging his tongue in as deep as he could, he saw her bite her finger as her breath came out in shudders, he savored the flavor of her passion before withdrawing, giving her a mischievous smile before assaulting her clitoris with his tongue.

Her hands shot from her face to the carpet in one smooth movement, gripping the fibres frantically as she cried out in ecstasy. She wrapped her legs around his head, pulling him closer, he took her delicious nub into his mouth, sucking gently, when she cried out again he sucked it harder, she screamed in her throes, Sans absentmindedly wondered if Papyrus was awake, he found he couldn't really bring himself to care. He licked her for awhile longer before gently lowering her hips back to the floor again. Her whole body was flush, she looked a million miles away, she was panting harshly and was still desperately clinging to the carpet.

"Please!" she begged, "Sans please!"

He leaned down to her ear whispering huskily, "Please what?"

She cried out again as his fingers entered her, "PLEASE!" her face was red, she didn't want to say it, "I need you inside me!"

He quickened the pace, "still not good enough." he stated teasingly.

"Please!" her breaths came out quickly, "Sans!" she whined his name.

Sans stopped moving altogether, loving the way she desperately grinded down on his fingers trying to get him to continue, "I'm going to need more then that…" he teased some more.

She cried out in frustration, "Please Sans!" he continued to stare at her, "Please fuck me!"

Sans decided that was good enough, he removed his fingers and discarded his pants before removing her panties from her ankle, they both sat there completely naked, he spread her legs leaning over her, rubbing her warm wetness with his twitching erection, her arms reached up weakly and cupped his face, he began to insert himself very slowly, "heh," he laughed, wondering if he would even fit, "You're so tight…"

She looked embarrassed, she pulled his face down, kissing him desperately, as if he were the only think keeping her alive right now, he continued to push gently, she moaned, tensing up slightly from the foreign feeling, he felt her get tighter. He broke the kiss, stroking her face gently, she relaxed a bit. He felt himself go all the way in and paused as he felt something break. She jumped, but she was still panting and still held the same desperate look on her face, only it seemed more pained.

"do you need to stop?" he asked, worried.

She shook her head, "there's no going back now." There was a wide grin on her face, he wondered what she meant.

He pulled out slowly, before pushing in again, she bit her lip, he paused again, her eyes met his as she let out a small laugh, "you're so big… I feel so full." She said putting a hand on her lower abdomen. His face heated up, he never expected to hear such dirty words come from her mouth. She pulled him in for another kiss, their tongues entwining as he continued his slow steady pace, taking in the moment. He broke the kiss to lean back and watch her, he loved the way she was biting her lip to hold in her moans, and the way her perky breasts bounced every time their hips connected. He kept the pace, but started to thrust in harder, he noticed she started to fall into rhythm with him, moving her hips down to meet his thrusts.

"faster!" she moaned, he happily obliged, quickening the pace, keeping a steady rhythm. Her hands shot to the carpet again, "Ah! Sans!" she called out to him, his hand found hers, lacing his fingers through hers, he held it down beside her head. His other hand held up her leg behind her knee, deepening the penetration, she gasped as he seemed to hit something, "Right there!" she cried desperately. he found a way to hit it again and she wiggled, "Aaah~" he loved the sound she made, he kept hitting that spot, eliciting more pleasurable sounds out of her, she could feel him swelling inside her. He could feel his orgasm coming, he tried to hold it off, determined to bring hers first.

That didn't take long, he kept up his pace and soon she was moaning loudly, "Ah, Sans, I'm gonna!" he mercilessly pounded her into the floor, she flailed slightly under his weight, "I'm coming Sans!" she called out his name as he felt her stiffen, pleasure hit her in waves as she rode out her orgasm, Sans still thrusting harshly into her.

She tightened up even more around him, he could feel orgasm approaching fast. "Frisk!" he called out to her.

"Come inside me!" her voice was breathy and exhausted, and he obeyed, a few more thrusts later and his semen was spilling into her, she felt warmth filling her up, she sighed as his pace slowed, he sat there for a moment, reveling in the act they just committed, before pulling out watching some of his white liquid spill out onto the floor.

He laid beside her, they took a moment to catch their breath before Frisk broke the silence. "Well, tonight was eventful." Sarcasm laced her words.

Sans laughed, "we should do that again sometime."

She hummed and nodded.

He sat up gently picking up her exhausted frame up off the floor and putting her on the bed, he crawled in with her pulling the blanket up to cover them, she turned, burying her face in his chest.

"oh and for the record…" he spoke.

"yeah?"

"as a skeleton I can conjure anything with magic."

She looked up at him, "Anything?"

"Anything."

She giggled, "I'd like to see that."

Sans was awakened the next morning to a tentative knock on his bedroom door, he hastily got out of bed and threw on his pants, answering the door shirtless.

It was Papyrus, he jumped at Sans appearance, still not used to the flesh. "Are you okay Sans? I heard and awful racket last night, but I didn't want to disturb you in case you were dealing with human things."

Sans felt guilty for not being quieter, "I'm fine Paps, it was nothing."

He looked skeptical, "are you sure because I swore I heard another voice in here…" he paused as he examined the hickey on his neck, "is that a bruise?" his eyes scanned over to his bed, seeing the sleeping form in it, everything clicked into place. His eyes met Sans', which avoided his, "You didn't."

Sans tried to hide his satisfaction, "Maybe I did." He replied, cocky.

Rage set into his face, "Sans! You get a human body and the first thing you do is take up the human habit of casual sex?!"

Sans hadn't realized how much Papyrus had picked up during his time on the surface, he shrugged nonchalantly "Wasn't the first thing… and it wasn't casual."

"Don't get coy with me! Who is that anyway?!" he was fuming, "don't tell me you went out and picked up a random stranger!"

"I didn't." he answered plainly.

"Then?!"

"That's Frisk."

Papyrus' jaw dropped, Sans grinned, "What?! How could you do that to her?!"

Sans shrugged, "she seemed to enjoy it just fine. after all, she did cause a racket." He said looking Papyrus in the eye mischievously.

Papyrus was about to explode, but was interrupted by a small hoarse voice, "Papyrus?" Frisk appeared behind Sans wearing his hoodie, which didn't seem to cover much. She leaned her head on Sans shoulder, squinting at the daylight, still half asleep.

Deciding to spare her any embarrassment, he dropped it, realizing they were adults and there was nothing he could do about their irresponsibility. "Good morning Frisk, I hadn't realized you were here." He said pleasantly, but shot a look at Sans, their conversation wasn't over.

She nodded sleepily, "I couldn't sleep last night so I came to visit Sans."

He nodded, "Well I'm going to go make breakfast." He stated, leaving promptly.

Sans guided Frisk back to the bed, laying her down before crawling in and holding her close. "I think I'll stop by tonight as well." She stated.

"I'd like that…" Sans replied, he sounded tired, he kissed the top of her head.

It wasn't long before they fell back into slumber, sleeping the day away.

A/N: A friend challenged me to write Undertale smut, specifically for Frisk and Sans, knowing that would be hard to write. so naturally I cheated and made Alphys create a machine using science and magical shenanigans. perhaps one day I'll write a non-human NSFW fic as I elude to near the end of this one, but it won't be until after I complete The Recital. hope you enjoyed!