When Toriel realised that Frisk hadn't had much in the way of formal education, she was- to put it mildly- put out, although very able to see the silver lining of taking charge of their education herself. At first Frisk had been okay with the idea... At first. But neither of them had really understood what it would include.
Frisk, of course, didn't understand that it involved sitting still for a long time, making a point of trying to absorb and retain information; and Toriel, while she knew she had to try to make it as interesting to Frisk as possible, didn't know them well enough to realise just how inclined to do their own thing they were.
In hindsight, she thought, she probably should have expected it. She'd never pushed the subject of the child's past, but it was clear they were alone in the world. They'd been used to living their way, without anyone to care what they did or where they went.

A breakthrough had happened when she'd been teaching Frisk about some human history she'd been reading about. She thought Frisk hadn't been paying attention, doodling instead- but that evening they'd called Undyne over and told her dramatic stories about what they'd learned- complete with drawings that, although rushed in the space of the afternoon, showed a lot of promise. Undyne enjoyed Frisk's "lesson", and it gave Toriel a lot of ideas.
She started to appeal to their apparent desire to doodle and draw and make up stories, and managed to make some headway; as well as pointing out that, if they learned maths and science, they could talk about it with Alphys. That helped a little- less than she would have liked, but it was a start. Art theory- which Mettaton decided he was perfect to help with, regardless of his own lack of education on the subject- was the only other subject sure to get them focused, and she decided to look into getting them some decent art supplies.

The other problem was that Frisk was... Well, they weren't ever naughty, exactly. They didn't make a point of breaking rules- although Toriel was starting to wonder if a certain inability to fully follow rules was just a thing human children did, despite her attempts to avoid any comparison. No, Frisk was just... It seemed like they almost 'forgot' they were supposed to do things, or just decide to wander off without telling her. Of course, given how eager they were to wander off in the ruins when she had just met them, that shouldn't be a surprise...
At least they tended to go to one of their friends, and they were good at letting her know. Still, it was exasperating, and she didn't know how to get through to Frisk without pushing them away. Her attempts so far had met with Frisk appearing penitent, but not really changing their ways.

Case in point- they were supposed to be getting ready for bed. They had, at least, done their homework, but of the child themself, there was no sign. She sighed and shook her head. At least every monster knew who Frisk was and would look out for them...

"...And when the fluffy bunny got home, everyone was happy to see him. The end." Sans closed the book and looked up. Papyrus was out cold. He put the book back on the shelf and left the room, taking one last look at Papyrus before he switched the light out and shut the door.
He whipped his head around at a creak on the stairs. A figure wrapped in a blanket was frozen in place half-way down the stairs. Slowly, they turned to look at him. Frisk's face was red, and they clearly didn't expect to be caught.

"Um. Hi."
Sans spoke quietly, heading down the stairs to join Frisk. "Hey, kiddo, what're you doin' here?"
"Oh. Nothing. I'll just... be going."
"Nothing." Sans took his hands from his pockets for the sole purpose of crossing them. Standing on a step above Frisk, looking down at them with arms folded, he somewhat reminded them of Toriel when she was disappointed in them. They squirmed, not meeting his gaze.

"I just, um. I was listening to you reading to Papyrus?"
"Were you." Sans' eyes creased with amusement, but he didn't give Frisk a way to escape the conversation. They twisted the blanket between their hands anxiously.
"W-well, nobody ever read to me, so- I mean... L-look, I'll just, I'll go. I can just get out of your way."
"Nah, that's fine Frisk, but why don't you ask Toriel to read to you? I'm sure she'd be eager to." His grin widened. "Over eager, maybe."
"N-no, I don't... want to ask. It's fine."
"So you want someone to read to you without asking? Alright kid, I get the hint- let me get a book."
"N-no! I-" Sans ignored Frisk's protests and they clamped their hands over their mouth as he went back into Papyrus' room. They felt so foolish and different scenarios flashed through their minds- leaving now, or making an excuse, or...

Or just letting him read to them. Sure, they were too old for it really, but Sans still read to Papyrus, didn't he? And he was the one to suggest it, even as they said otherwise. Maybe he wanted to. Maybe they should go along with it.
They nodded and headed back up the stairs as Sans came back out. He nodded, book in hand, clearly not wanting to speak so close to Papyrus' door. That left him in a bit of a bind when Frisk, instead of going downstairs to sit on the sofa, went to his bedroom door. They put their hand on his doorknob and looked at him, head tilted to one side quizzically.

He narrowed his eyes. Frisk widened theirs and pointed to his door. He sighed and headed over to them- which they took as permission, and opened the door. He rolled his eyes, wishing he'd locked it. Oh well.
Frisk ran over to his mattress and sat on it, still wrapped in their blanket, and kicked off their shoes.
"Kid- what are you doing?"
"I thought you wanted to read me a bedtime story?" They crawled onto his bed- his covers were a mess, but they curled up next to the wall and arranged the blanket around themself.
"Didn't realise you were gonna steal my bed for it."
"Well it wouldn't be a bedtime story without a bed, would it?" Frisk looked at him over the top of their blanket, all innocence and wide eyes. Sans sighed. The kid was even harder to say no to than Papyrus.
"Alright, I can sleep downstairs tonight."
"What? No!" Frisk sat up, the blanket falling off and pooling in their lap. "That's not fair on you."
Sans stared at the kid. He stared at the bed. He realised what they had expected and groaned, letting his head fall into his hand.
"You wanted to spend the night in my bed. With me."
"What's wrong with that?"

This time, it definitely wasn't fake innocence. Frisk was just a kid who wanted to spend the night with their friend. It wouldn't be long before they started to think differently, but- Sans realised- they hadn't had much of a childhood, if his guess was correct, and his guesses usually were. Frisk was, for the most part, mature beyond their years; perhaps forced to grow up too fast? And they didn't have any family, in fact given how much trouble they were having adjusting to having people there at all, he suspected they'd never had much stability. Of course they wanted to cling to their friends while they could. He raised his head, nodding, giving Frisk a grin.
"Let me go and change, kid."

When he returned, in a battered t-shirt, pyjama shorts and fluffy slippers, Frisk had straightened out his covers- somewhat- and had placed their jacket, neatly folded, on the cardboard box that was the closest he had to furniture here. They were lying back against the wall, using the blanket as a pillow, and gave him a tired smile.

"Alright, kid. Here you go." He climbed into the bed next to Frisk, who peered at the book in his hand.
"'The Sleepy Kitten'? I, uh... I think I'm probably not the target audience for that."
"Really? 'Cause I think it's purr-fect for you."
Frisk groaned. "It's nothing pawsonal..."
"You sure? 'Claws it sort of sounds like it is..."
"No no, you've convinced me. You're pretty purrsuasive."
"Alright, after this at least take a cat-nap."
"I can't keep up, Sans. My attempts at puns are... Catastrophic."

Sans actually laughed at that one. He ruffled Frisk's hair. "Okay, kid. I can't top that. No more kitten around, you wanted a bedtime story- this is what we've got."
"Okay, okay." They pulled the covers around their face, peeking over them at him. His grin widened slightly and he settled in and opened the book.
"There was once a little kitten..."

The book didn't have the same effect on Frisk as they tended to on Papyrus. They kept their eyes fixed on his face as he read, and once Sans was done, they looked perhaps less sleepy than they did before he started. He tossed the book to one side.
"Better, kid?"
"Y-yeah. Thanks." Frisk fidgeted, hugging their knees to their chest, before wiggling over to lay their head on Sans' shoulder. They managed to stay still for a few seconds before grumbling and fidgeting some more.
"What?"
"I can't use you as a pillow," they whined. "You're too hard!"
"Yep, that'll happen when you lie on a skeleton, kid."
Frisk grumbled and lay on their back. Sans raised himself on one elbow to look at them, then shifted around.
"Welp, guess if you can't use me as a pillow..."
"Gah! Sans, no!" Frisk started giggling as he rested his head on their belly. "Noo, come on... You, you numbskull!" They grabbed his pillow and plopped it on his face to hide his smirk. "I'm not a pillowww... I couldn't sleep like this even if I was bone tired!"
"Heh." Sans took the pillow off his face and peered up at Frisk. "You're getting better at this, kid." There was a pause, then he sat up and gave Frisk a hard stare. "Say, does Tori know you're here?"
"Oh, well... Probably? I mean... Where else would I go..."

Sans let out a sigh and got out of bed. "I'll go call her."
Frisk managed to hide their expression until he was out of the room. They'd been so focused on enjoying being with their friend, with the attention, that they forgot all about the fact that they had a place they were supposed to be. They kept doing that- it was so easy to forget. They felt a pang of guilt. What if Toriel told them not to come back, one day? What if everyone did? They fidgeted, feeling worse and worse, until Sans came back.

"Is she okay?" were the first words out of their mouth as soon as Sans came back.
"Yeah, but-"
"Is she mad at me?"
"She-"
"Oh, is- is she going to kick me out?"
"Kid, shut up." Frisk did so and Sans sat on the bed, looking at them. The light in his eyes was dim, barely visible- a sure sign of how serious he viewed this. "She's fine. I don't think she's ever going to kick you out. But you oughta let her know where you're going."
Frisk hung their head. "I know, I- I know. I just... I forget, I-"
"You're not used to having to answer to anyone?" Frisk nodded and looked up at him through their hair. "Yeah, I get that, buddo. Just... Try, okay?" Frisk nodded again.
"I- yeah. I will. I mean, I have been. I just..."
"You don't have to explain it to me, but maybe you should explain it to her. Man... Sometimes we'd talk through that door between jokes, about family, and she would sometimes tell me to sleep more, or eat better, or tell me to make sure I have warm clothes... But you, you're her kid now, Frisk. She's gonna worry anyway, but you don't have to make it harder for her."

Frisk nodded and lay back, staring up. Thoughts swirled around in their head, but... They'd burdened Sans enough for one night. They wanted to try and remember the good in this night, not weigh it down with more of their so-called problems.
He seemed to guess, though, as he turned the light off and got into bed. "Hey, Frisk?" They looked over. "She's not goin' anywhere. Neither am I."
There was a pause. Sans could just see Frisk's smile in the low light.
"Thanks."

Sharing his bed with the kid wasn't too bad- especially compared to sharing it with Papyrus. But Frisk did wake him up once. The whimpers, begging and cries in his own dreams chased him out of them, following him to wakefulness. He shook away the remnants, a combination of things that had happened in another time and things his mind dredged up; but the noise continued and he remembered Frisk.
He rolled over. The child was curled up, one hand balled into a fist clutching their hair; and although no words were discernible, the fear behind them was. He stroked their hair, trying to soothe them; trying to gently untangle their hand. Slowly the whimpering slowed, then stopped; their fist uncurled. They shifted and he looked down at their face- one eye cracked open, full of sleepiness, and that hand took his and pulled it to their chest before they settled back in. Deep breathing told Sans that they fell back asleep instantly; but he stayed awake for a while after he managed to get comfortable, feeling the heat from their small hands and watching them. He wasn't sure when he drifted back to sleep himself, but Frisk didn't seem to have any more nightmares while he watched- and once he finally got back to sleep, neither did he.