Sans has decided that his favorite AU is Underswap. His favoritism doesn't come from the scenery or the flipped environment. He loves the AU because he loves being a Swap!Sans.

Each time he wakes up is an entirely new experience. Today, Sans wakes up with a snap of clarity and energy. No morning grogginess for him! His thoughts come in rapid bursts with an eagerness to start the day. This isn't Sans's own behavior, but the Swap's. Sans becomes whoever he wakes up as. His own thoughts are drowned out by the cares and worries of his host.

Throwing off his blankets, Sans practically leaps out of bed. His blue pajamas are a little too big for him, and he nearly slips on one of the pants' legs. Sans yelps and grabs onto his nightstand to keep himself upright. He giggles at his own clumsiness. Sans is more careful this time as he gathers up his clothes and heads for the bathroom. A little spark of pride runs through him every time Sans sees his "battle body". He made it himself after Alphys, Captain of the Royal Guard, agreed to train him.

Sans closes the bathroom door behind him and pushes a small step stool in front of the sink. He's too short to see much of himself in the mirror without it. Sans gets lost in thought as he starts brushing his teeth. There's always so much he wants to get done in the day, but so few hours to work. Sans is distracted from his planning when he locks eyes with himself in the mirror. His big blue eye lights bore into those of the reflections.

This is wrong. Sans looks away and shakes his head. It's easy to forget that he's just passing through. Still with the toothbrush in his mouth, Sans lets go of the handle and checks himself. A small, white Monster soul appears hovering over his hand. It shows no sign of its temporary tenant. Without any evidence, it's hard to not wonder if he's even real. Sans almost prefers to lose himself in the body of each new day rather than be plagued with these kinds of questions.

It's easy to put his unease to the back of his mind when in a Swap's body. They're so cheery and optimistic that Sans quickly forgets about what had just been bothering him. He un-summons his soul and quickly spits out his mouthful of toothpaste before getting dressed.

Sans shoves the foot stool back in its place under the sink once he's done with the mirror. Knowing his brother, that lazy bones would trip over it in half-asleep stupor. Sans stares at the closed cabinet with his hands on his hips for several moments as the thought sinks in. Papyrus! Sans rockets out of the bathroom and down the hall. He nearly forgot to wake up his older brother! Sans dimly recalls that some Swap AUs place Papyrus as younger, but that's not the case here.

Barging into the room, Sans takes a running leap and launches himself into his brother's bed. Papyrus lets out a quiet "oof" when Sans lands on his chest. Their heights vary, but this Swap is comically short. He's only the size of Papyrus's torso, and sits comfortably on his brother's chest.

"Pappy! It's time to wake up!" Sans wines, bouncing up and down. Both of them know that Papyrus has an alarm clock that he could easily program. They both also realize that they would much rather start their morning like this.

"Five more minutes, bro," Papyrus says lazily. He pulls a hand out of his blanket cocoon to rub Sans on the head. The elder is so wrapped up in blankets that not even his face is visible. Sans happily leans into his brother's hand like a cat, but then snaps out of it.

"No more sleeping! You're gonna be late and miss breakfast," Sans complains. The correct dialogue and mannerisms come easily to him. Papyrus will never know that there's a second mind in his brother's body for the day.

"How about we just go to Muffet's today? I'll pay," Papyrus replies, more awake now.

"Absolutely not! Such sugary food is not suitable for a morning meal!" Sans crawls off of Papyrus and gets off the bed, posing in a heroic fashion. "I, the Magnificent Sans, will cook a nutritious and delicious breakfast for my dear brother!"

With that, Sans runs out of the room, giggling cheerfully all the way. He sprints down the hallway and runs down the stairs just as quickly. On the third to last step, the tip of one of Sans's blue boots catches the edge of a stair. He trips and falls down the last few steps before ending up on the living room floor. The one thing that seems to follow Sans from body to body is his clumsiness. He sits up and shakes his head, dazed from the fall.

"Sans! Are you alright!?" Papyrus shouts. Upon hearing the crash, he had taken a shortcut downstairs. Papyrus picks Sans up off the floor to make sure that nothing is broken. Although his little bro is all grown up and able to take care of himself, Papyrus raised Sans after their father disappeared. He'll always be looking out for Sans. Papyrus hugs Sans to his chest after finding no obvious injuries.

"I'm okay, Pappy, really. It was only a little fall." The real Swap might have immediately started to squirm and try to get away to start breakfast, but Sans lets Papyrus hold him a moment longer. No matter the AU, Sans loves his brother. That much is always real. Sans also knows that Papyrus isn't scared of the fall as much as he's scared of the resets. Swap doesn't remember the resets, but Sans knows what they are. Today is the day that the human falls. It always is. Papyrus must already be worried about who he'll meet outside the doors of the Ruins. Sans squeezes his brother a little tighter. It's all he can ever do to comfort him. Sans does his best not to dwell on the resets.

Breakfast consists of Sans's famous tacos, breakfeast-afied, of course. By the time Papyrus comes back downstairs after getting ready, Sans is plating up the sausage and eggs that he warmed up on the stove. The two of them eat together and go over their plans today. Sans has training this morning with Alphys, but plans to meet up with Papyrus later to recalibrate the puzzles between Snowdin and the Ruins. The Dogs are usually too forgetful to manage.

The brothers part ways after cleaning up. Sans heads towards Waterfall, picking up speed until he's running through town. He laughs and then waves to any monster he passes. Running is a good way to warm up for training and take the edge off the whirling energy that always seems ready to burst in a Swap's body. Sans feels like he could run forever!

Waterfall is breathtakingly beautiful. It's Sans's favorite place in any AU. His footsteps echo loudly in the narrow tunnel as he runs around deep water and through tall grasses. Echo flowers whisper sweet wishes, and glowing fauna make the low cave ceiling sparkle. Sans is so caught up in his surroundings that, once again, he trips. He yelps as a rock catches his foot. Sans falls flat onto his face as he enters the cavern where Alphys's house is built. Sans lays there for a moment in embarrassment with his face pressed up against the cold stone.

"Sans? Why are you on the ground?"

Sans bolts up into a sitting position when he hears the voice. Alphys had stepped out of her house since it was close to their usual training time. She found a small skeleton sprawled out on the ground across the small cavern.

"I-I just," Sans stumbles, face heating up in a blue blush out of embarrassment. He looks up to Alphys, and would rather not have her see him acting so clumsy. "I just fell down a little."

Alphys snorts.

"It's not funny!" Sans gets to his feet and stomps his foot in frustration. "Everyone trips!"

"Sure," Alphys chuckles again, but this time more towards Sans's overreaction. "Let's start practice, yeah?"

Sparing is one of Swap's favorite games, so it's currently Sans's favorite, too. He's not sure if he actually likes it or not. The rush of adrenaline mixed with the friendly competition creates a feeling like no other. Sans has to keep reminding himself to fight like Swap does. He's lived in a thousand different bodies each with their own unique fighting styles and techniques. It's easy to fall into the natural rhythm of Swap's fighting style, but every now and then, Sans has to stop himself from throwing a punch or summoning a Gaster Blaster. He lets himself take hits just like what's expected of him.

By the time Sans and Alphys are done with their sparring session, they're both panting and sweating. Sans's boundless energy is the only reason that he can keep up with the Captain of the Royal guard. The two stare each other down from ten feet apart. Finally, Sans dissolves the two long bones he had been using to spar with. He falls to an exhausted heap on the ground.

"Fine," Sans pants, "Just for today, the Magnificent Sans will concede...... defeat." He ignores the fact that this is how their practices almost always go.

"Don't feel bad, Sans. You're getting better all the time," Alphys tries to console him. The two of them sit in a comfortable silence for a few moments while catching their breath. Sans abruptly bolts upright.

"Oh! I have to go meet up with Pappy!" he shouts, jumping to his feet. "Thank you for the practice! See you tomorrow!" Sans waves before turning and sprinting away at full speed.

Alphys looks after him with a bewildered expression. "How can he still run?" Now that Sans is gone, Alphys falls to one knee, exhausted from trying to keep up with the smaller's limitless energy.

Sans only makes one stop at his house to down a cup of water before continuing on his way to Papyrus. Hydration is of the utmost importance! The road out of Snowdin leads right into the forest beyond. Walking this time, Sans follows the path to his brother's sentry station. He can hear Papyrus talking from around the corner before Sans sees him. A chill runs down Sans's neck. He wants to spin around and ignore the script, but that's not what he's supposed to do. Sans hides his worry behind a cheerful smile before approaching his brother.

For the next hour, Sans says what he's supposed to say and acts as oblivious as he should be. The problem is that he's not oblivious. Every minute that creeps by is a moment closer to the end. Sans knew it the moment he saw the human. A pit of fear builds up in his chest, but it's not for himself. Papyrus remembers every reset. He remembers each time that Swap has been killed. Sans doesn't want his brother to see it again. It pains him, but Sans believes that there's nothing he can do. He has to follow the fate of this AU.

When they exit Snowdin, Sans meets the human on the road into Waterfall. They fight, but Swap's skills are no match for a human that's already killed him dozens of times. The damage starts to build up. Sans can feel himself weakening. Papyrus......

"I still...... believe in you," Sans mumbles, lacking conviction as the human walks towards him. He has to fight with himself to stand still. Sans closes his eyes at the last moment so he doesn't have to see it coming.