The thing is, Pap's a cryer.
It's totally normal, for a babybones his age—sans had done his homework, okay, he's not a totally shit caretaker. He'd pored over dozens of parenting guides those long, terrified nights he spent jittering-awake at the foot of Papyrus' bed having downed a pot of coffee not an hour before, every nerve he doesn't have strung tight, just waiting to be plucked by the closing of the front door and the arrival of—
It might even be a good thing, he thinks sometimes, because as bullheaded as the kid can be, sans at the very least always knows what's happening inside his skull. He knows when pap's upset, or sad, or furious that his dad didn't make it home for dinner again because he broadcasts it at truly alarming volumes. Even if his temper tantrums are exhausting, he’ll take ‘em, because they’re a damn sight better than the way sans' brain just sort of...goes offline sometimes.
There's a lot that scares him, but this, this is the absolute worst.
He'll blink awake under the dinner table, curled up in a ball at the end of Pap's bed, shivering on the back porch while Pap pounds at the glass and wails for him to be let back inside, and have absolutely no idea how he got there.
He doesn't tell Gaster. He should, he knows anomalous behavior should be immediately reported, so that it can be appropriately documented and studied, but the stomach he doesn't have twists itself into tight knots at the thought of trying to explain it to his creator. Of trying to put into words, into case notes he bleak horror of having blinked and opened his eyes to find the whole world has fast-forwarded around him. He shivers. He tries desperately not to think about it.
And it's not...Pap doesn't throw fits or anything, it's not really a temper tantrum, it's more that he just sort of starts tearing up when he's really furious. On top of that, he has the added bonus of absolutely loathing his own body's betrayal, which makes him even angrier, which makes him cry more.
It's a vicious cycle.
Sans figures out by age fourteen r
ight? does he go by the age of his bones, or the years he’s been awake? that Gaster probably wasn't expecting much from him as a test subject.
Sans can't blame him, really. He knows enough to understand how rarely it is that an experiment goes right on the first handful of tries, and…okay, he's not perfect, he's not even very close, but he's coherent and he's sentient and he doesn't require much more maintenance than a real monster. He's a success, which thrills Gaster more than anything, he thinks, this walking, talking tribute to his own brilliance trailing after him like a pitiful, huddled-down shadow. Sans strokes his ego simply by existing, and that's usually enough to grant him at least a begrudging place to sleep on the couch. Some nights, there are even blankets.
It's damn lucky he turned out as well as he did, actually, because there's a severely limited supply of vessels Gaster could have used. One missing corpse smuggled from the castle in the dead of night is risk enough—though sans wonders sometimes if it was anything more than his own masturbatory dramatics that made Gaster choose the dusty, poisoned bones of the former princeling instead of literally anyone else.
It’s creepy. He’s creepy. He wishes often that Gaster had at least chosen a bigger one, maybe, because his body is awkward as a result, unbalanced in its pubescent half-development. He supposes, it could be worse. It's practically a miracle he can even talk, never mind his current career as Gaster's favorite lab assistant.
Sometimes, Gaster even trusts him to keep an experiment running on his own for a few hours while he oozes upstairs to have dinner with his son. Sans stays in the basement, of course. He knows the rules.
dogs don't eat at the table.
He scratches absent at his collar with one clawed hand, and, head tilted in consideration, finally flips a switch on the control panel in front of him. A new line hums to life on the display readout, this even, spiking pattern traced in hot pink. He watches it dully, heavy-lidded, praying it will do something interesting, something he can focus on besides the clink of forks on ceramic plates and the heavy smell of roasted garlic drifting down from the staircase. It doesn’t, of course. His nonexistent stomach grumbles loudly.
He knows the rules.
His mind is the only thing he has of any value to his adopted father, anymore. He's seen Gaster's original sketches for Project S4-N5, and he’s…well, he’s the disappointing sibling right out of the gate, to say the least.
He's maybe a third of his projected size, likely not much bigger than the child his body once was. His blunt, awkward fangs are nothing like the intended tusks, his balance on two legs precarious and tiptoed, rather than the low, barrel-chested stance he should have had.
The tail is limber enough that it’s easily wrapped surreptitiously around one leg, at least, and while it makes it difficult to walk sometimes with no counterbalance, he's eternally thankful he looks normal with most of his body hidden under heavy, baggy clothes, his clawed toes shoved into bulky, too-bright sneakers.
He's pretty lucky, he supposes, that his adopted family looks so similar to the remains of a human body. He passes for one of them, anyways, as long as he doesn't open his mouth too wide, as long as he doesn't let his burning left eye flicker eerily to life. It’s probably the only reason he’s allowed in the house with them. Or out if it, come to think of it.
He’s seen the shed and the straw bedding for a pet they don't have
And hey—if he keeps his hands in his pockets, he and Papyrus could actually be mistaken for brothers, maybe.
His own stuttering magic—borrowed and incomplete without a real soul to root into, Gaster says, and he’s says it like he’s deeply disappointed—gouges at his brain like an ice pick to the skull, most days.
He's relieved to find that the whiskey in the dusty depths of Gaster's coffee cabinet works to soothe the hurt, despite his lack of any apparent nervous system. He's rarely afforded painkillers even so, for fear of their interference with the cocktail of chemicals pumped through him occasionally, but Gaster never says anything when he turns up for his shifts in the lab reeking of the inside of Grillby’s, as long as he can stand on his malformed feet.
This particular coping mechanism eventually catches up with him. Sans isn't surprised at all. Truth be told, the peace he felt when he was well and truly gone, the easing of the ever-present tangle of the dark and slick thing in his ribcage, the unusual sensation of warmth—it was such a laughable relief that it almost made him uncomfortable.
Gaster strikes him often but for this particular transgression, he only needs to do it once.
It happens on a night he'd completely forgot he was meant to be watching over his charge while Gaster worked late in the basement lab, mostly because...well, he could have sworn he checked the calendar on his phone before he cracked his first beer, and Papyrus was meant to be in soccer practice until the evening, at least.
Sans has been given the day off, so to speak, and spends it in the blessed silence of their empty house, humming quietly to himself as he cleans.
It's nice. It's calm. It's too easy to lose track of time—and liquor—this way, elbow-deep in warm, soapy water, cheekbones pleasantly flushed as he works lazily at the stained grout of the kitchen floor.
He likes cleaning.
He likes—well, maybe he just likes routine, actually, maybe he just likes the buzzing way his brain bleeds off when his hands are occupied.
He likes the quiet of the house when Papyrus is at school and Gaster is at work, and he likes the antiseptic burn of bleach in lungs he does not have. He likes being able to breathe without that constant, trembling terror of making a mistake. His father is nothing if not vigilant in a lab environment, but when it comes to their home, he is never inclined to do much to help.
Point is, sans is a six-pack of shitty beer and three generous tumblers of whiskey in when abruptly, the front door slams open hard enough to embed the knob into drywall, and he no longer has the house to himself.
So, alright, here’s the thing. Pap’s a little too loud. A little too brusque. He rubs kids the wrong way, sans guesses, tending to order his friends around like he's somehow commandeered the playground, and crying furiously when he doesn't get his way and...kids are like puppies, right? They play fight. That's normal. That's socializing. Papyrus snaps, they snap back. It's fine.
It's fine until Papyrus comes home with a black eye, anyways, tear-tracks smeared all down his dirty cheekbones, and the collar of his t-shirt torn in three places. He’s bright orange with rage, huffing like a winded horse, and sans’ admonishment of hey, be careful with the wall dies on his tongue.
Papyrus’ right eye is deep red and angry, swollen nearly shut, the ridge of his cheekbone bruised to match. Whoever hit him had a hell of a hook, or else they’d taken several swings at his face, and from the way he’s standing, sans is willing to bed he’d find more of the same under Pap’s clothes.
When sans reaches out to touch his eye, horrified, with a low, mournful "oh, Pap," Papyrus snarls at him and slaps his hand away viciously, like sans had been the one to give him the shiner.
"Don't," Papyrus growled. "Just. Don't. I can handle it, fuck off."
“kay,” sans says, drawing back. “but you don't have to handle it, pal. that's what I'm here for. remember?"
Papyrus shoves past him with a, "How could I possibly forget, you’re literally wearing a collar,” spat over his shoulder as he storms upstairs. Sans watches him go, and not ten seconds later, he hears Papyrus’ bedroom door slam shut, the lock sliding home with a heavy thunk followed by the creaking of springs that meant his charge had thrown himself bodily onto his bed.
He stares after Papyrus, eyelights dim and considering, for maybe a minute in total, swaying only a little.
He doesn’t lock the front door behind him when he leaves the house and makes a beeline for the playground.
what does a dog need a key for?