Exotherm

“I’ve had enough of this,” Aizawa scowled, eyes flicking back and forth between the two teenagers wrapped up in his capture scarf. “After this long I thought maybe you’d have learned to work together. So for the next twenty-four hours, you will not leave each other’s side. You will sit together in class, you will cook dinner for all your classmates together tonight, you will eat said dinner together, you will sleep in the same room, you will shower together if that's what it takes to work things out. At the end of the school day tomorrow, if you have managed to not kill each other, I will consider letting you have your independence again. If you are still fighting, maybe we’ll have to consider more drastic methods.”

Katsuki stuffed his hands in his pockets, looking away with a huff, but Izuku conceded a faint nod.

“Sorry, Sensei,” he mumbled. “It won’t happen again.”

“We both know that isn’t true, Midoriya.”

“I’ll try my best,” Izuku amended. “We both will, right, Kacchan?”

“Don’t put words in my mouth, nerd.”

“Kacchan,” he whined. “Work with me, here!”

“Twenty-four hours,” Aizawa reiterated. “You’re dismissed.”

They walked back to the dorms in silence, refusing to even look at each other, until they reached the front doors and Izuku grabbed Katsuki’s sleeve lightly.

“Let’s just get through this, okay?” he said softly. “I’m sorry, the fights were stupid, let’s just get through this day and show him we can be sensible, okay?”

“Fine,” Katsuki huffed, pulling his arm away. “I’m in charge of dinner, just do what I say.”

Izuku almost argued, but he clamped his mouth shut just in time, nodding awkwardly instead. He was used to following Katsuki’s lead, after all, and he was big enough to admit that Katsuki was probably the better cook out of them. Katsuki looked at him stubbornly, eyes narrowed, just waiting for him to complain about it, but Izuku just forced a wobbly smile in return.

“Sure, Kacchan,” he choked out. “Happy to help.”

He followed Katsuki into the kitchen, and as they worked through chopping vegetables and steaming rice, the tension between them began to settle. The methodical, repetitive nature of cooking had always seemed to calm Katsuki down, and Izuku felt like maybe it was rubbing off on him a little – just sitting back and doing the simple tasks Katsuki assigned him gave him a chance to zone out and get out of his head.

In hindsight, the argument had been partially Izuku’s fault. He was big enough to admit that, even if Katsuki might not be. The silence between them was deafening, though, so hard to break just to say “I’m sorry” when he knew Katsuki hated those words anyway.

Katsuki had always been about actions rather than words, so maybe he’d be better off showing Katsuki he was sorry. They could go up to one of their dorms with their food, could hang out in silence as they did their homework, staying out of each other’s way, and he would even let Katsuki take the bed overnight.

Of course, it was partly Katsuki’s fault too, but he wasn’t really expecting an apology for that. Kacchan was Kacchan, after all.

“Here,” Katsuki grunted, shoving a bowl in his hands. “Where do you want to eat?”

“Oh! Um, thanks. I thought we could go to one of our rooms, if you want? I have some homework to do.”

“Fine.”

He grabbed his own bowl, yelling at the living room to come eat their food, and Izuku followed as he stalked off to the elevators. They stood beside each other in silence, staring at the floor buttons, until Izuku finally worked up the courage to ask.

“Do you want to eat in my room or yours?”

“Yours.”

“Oh, really? Okay!”

He pushed the button for his floor and the doors slid closed, the silence falling again as they moved up through the floors. He dug his key out of his bag when they got close, and Katsuki glanced at his hand, snorting at the beaten-up All Might keychain hanging from the small collection.

“Same one?” he asked.

“Yeah! I’ve had it so long, I’d feel naked without it. Do you still have yours, too?”

“Mm.”

Izuku smiled to himself, running the battered metal charm through his fingers as he led the way to his door, waving Katsuki in ahead of himself to find a spot to sit. He couldn’t remember the last time Katsuki had been in his room, suddenly finding him self-conscious about it all – how messy it was, how old and faded most of his posters had gotten, how he hadn’t made the bed in a while.

He opened the balcony door to let some fresh air in, letting Katsuki take a seat at his desk and setting up his own stuff on the bed.  They were still in their school uniforms, called straight to Aizawa after their disaster of a heroics class, so Izuku moved to his closet to change. He expected Katsuki to avert his eyes, but instead he felt them burning into his back, glancing over his shoulder to see what the big deal was. Sure enough, sharp red eyes were looking him over, and Izuku raised an eyebrow as he picked up a t-shirt.

“New scars,” Katsuki grumbled, before Izuku could say the words. “Haven’t seen those ones.”

“Oh,” Izuku hummed, craning his neck to see the white slashes he’d obtained a few weeks prior. “Yeah, I guess not. It’s no big deal, just a fight during work experience.”

He pulled his shirt over his head, and Katsuki drummed his fingers on the desk for a moment, lost in thought. Izuku took the opportunity to change his pants, getting comfortable on his bed beside his textbooks, and Katsuki sighed as he scooched his chair a little closer.

“You probably need help with this English shit,” Katsuki huffed, flipping to his notes. “I guess I’ll help, since I’m here anyway.”

Izuku blinked at him, then his lips stretched into a smile, nodding quickly before Katsuki could change his mind.

“Thanks, Kacchan!”

Maybe Katsuki felt a little bad after all.

─────

Somehow, they got through the day without incident. They’d slept in Katsuki’s room overnight, after he’d helped carry Izuku’s mattress down to go on the floor, and by morning things were practically back to normal – still a little awkward, considering they had to walk to and from the bathroom together and all, but they’d gone for a morning jog in the crisp, fresh air; had made breakfast together, without arguing about nutrients versus flavour; had even walked to class together without a single argument. Aizawa seemed almost impressed to see them co-operating so well, he hadn’t even felt the need to address the subject.

Their last class of the day, the last moment before freedom, came down to a science class across the school in one of the purpose-built labs. They didn’t get to use them often, maybe once a month if heroics didn’t get in the way, but it was always an exciting day when they got to go play around with chemicals. They even got to choose their own lab partners, got to have some fun screwing around with a friend, but Izuku supposed he didn’t get to make that decision this time. He sat down near the front of the class, like Katsuki preferred – and he called Izuku a nerd – and sure enough, Katsuki sat down beside him, picking up their instruction sheet to read ahead.

“What the hell is an elephant’s toothpaste?” he grumbled, frowning at it. “How is this gonna help me pass an exam?”

“Not everything is about exams, Kacchan!”

“Whatever. Let’s get this over with so we can stop following each other around all the time.”

The teacher gave them a quick explanation, and Izuku leaned over to join in reading the instructions. It all seemed simple enough, just a few ingredients to throw together in the right order, and he smiled as he picked up a little bottle of green food colouring.

“Let’s make it green, Kacchan!”

“Haah? No way, orange is way cooler.”

“But we always use orange when we work together! Can’t I have green just this once?”

“Because green is gross. It’ll look like snot.”

“If your snot is emerald green you need to see a doctor.”

Katsuki huffed, glancing at the directions again, and finally conceded a nod.

“Alright, let’s half everything,” he suggested. “We’ll get the smaller flasks and we can make one of each.”

“Oh! Yeah, okay!”

“And then they can fight,” Katsuki continued, with a sharp-toothed grin. “I bet the orange will kick the green’s ass.”

“No way! It’s simple colour theory, Kacchan! Green is darker, so it’ll overwhelm the orange!”

“Not a chance.”

Both grinning now, they ran off to find their ingredients in the store cupboard, delighted to find there was more than enough of everything for them to make two full-sized concoctions instead of the wimpy half-measures. Izuku gloated over the green-tinged dish soap, already tipping the scales in his favour, but Katsuki just snorted at him, pulling his hair playfully.

“Because you need the advantage, nerd,” he smirked. “You’re already coming from behind, they’ve gotta give you some help.”

“I don’t need help! Green is gonna kick butt!”

“Have you ever even washed a dish before? The bubbles go white.”

“It’s still a sign!”

“Do you have sixty-seven protons or something? You’re acting like a real Ho.”

Izuku clapped a hand over his mouth to stifle his unattractive snort, but Katsuki seemed strangely delighted to hear it, shoving him fondly as they headed back to their table to get set up. The rest of the class seemed fairly docile, calmly going through their checklists and getting their equipment ready, but Izuku felt like a toddler on a sugar high, giggling like an idiot as he drowned his flask in green food colouring, swirling it around and tipping out the excess in the hopes that it might coat his mixture nicely.

“Do you know any good jokes about sodium?” he asked, handing Katsuki a bottle of dish soap.

“What? No, why? Do you have one?”

“Na.”

Katsuki froze for a second, staring in silence, but when a grin tugged at the corner of Izuku’s lips, he finally let out a groan. Izuku giggled into his hands, trying his best not to disrupt the rest of the room, but he knew it was too late by the way eyes kept darting toward them, shooting them concerned and confused looks.

“I’m surprised,” he continued, giggling through the words. “That one doesn’t usually get a reaction.”

“Very noble of them.”

“Are you two seriously making chemistry puns over there?” Uraraka asked, dumbfounded.

“Only periodically.”

It was Izuku’s turn to groan this time, missing the way Katsuki grinned at him for it, and Uraraka just sighed, shaking her head as she went back to her own equipment.

“You’re gonna kill someone at this rate,” Izuku grinned. “If they don’t kill you first.”

“You gonna help me barium?”

“Kacchan,” Izuku whined, hiding his face in his hands. “That one was so bad.”

“Says you, Mister ‘Na’. Idiot.”

“I’m not exactly in my element when it comes to puns.”

“That one was a little better.”

“Thank you.”

“Not great though.”

“I’ll keep working on it, but all the really good ones argon.”

Katsuki let out a short, sharp bark of laughter, making every head in the room whip in his direction as he slammed a hand over his mouth, but it was too late – everyone had heard it. Izuku in particular was gleeful over it, giggling up a storm, his face bright red as he struggled to keep enough oxygen in his system to function.

“Alright, I’m glad you’re having fun, boys, but it’s time to get on with the experiment.”

“Sorry, Sensei!” Midoriya squeaked, biting down on his lower lip to keep the giggles at bay. “We’re working on it, I promise!”

He poured the dish soap into his flask, watching Katsuki do the same, adding a few drops of orange to his flask along with it, and then they took turns adding the white powderinto water, swirling it up and pouring it in carefully, trying their best not to spill it as they poured through the narrow openings. Izuku put his safety goggles on, picking up his little beaker of hydrogen peroxide, and Katsuki glanced over to confirm he was ready.

“On three?”

“On three!”

“One... Two...”

They poured their water in together, taking a step back from the table, and immediately the flasks erupted with coloured bubbles, shooting straight up at the classroom ceiling. Katsuki cackled as they fired off, but Izuku watched in horror as they splatteredall over the ceiling, foam flying off in every direction amidst squawks from the classmates around them.

The reaction seemed to go on forever, covering the table in green and orange foam and still continuing to fire at the ceiling. When Izuku looked around, he saw much calmer foams spilling onto metal trays they’d put underneath for ease of clean up, a dozen gaping mouths staring at the mess Izuku and Katsuki had created.

“Boys,” the teacher sighed, pinching at the bridge of her nose. “Please read the instructions carefully.”

Izuku picked up the laminated page, flicking some foam off it and taking a moment to read it again, frowning at the words. Everything seemed right, except-

“The flasks,” he whined. “Kacchan, we used the wrong flask! No wonder it was so hard to pour things in!”

“And the peroxide,” Katsuki mumbled. “I got the strong stuff.”

“What?! It’s not like you to make that kind of mistake!”

Katsuki smirked at him, and Izuku could only stare back, jaw falling open as it all clicked together.

“You did it on purpose,” he whispered. “Kacchan! Why?! Now we’re gonna have to clean this all up!”

“It was fun though, right?”

“W-Well yeah, but-”

Katsuki shrugged, and Izuku fell silent, pausing for a moment before he cracked a small smile.

“It was a lot of fun,” he admitted. “And I’m pretty sure green won.”

“Like hell it did! Orange kicked ass!”

“And we’re gonna have to clean this up anyway, so we may as well do it again.”

Izuku laughed this time, but no one moved to stop them, so he nodded. He grabbed a clean flask, and this time he took charge of both bottles of colouring, carefully spreading green over one side, and orange the other. Katsuki got the catalyst ready, and with a big grin he dumped it into the flask, setting off another massive explosion of foam. This time it was mostly white, but each side had a big stripe of green or orange, and Izuku beamed as he watched their colours burst and swirl and fall together. The concoction was radiating heat by the end, but Katsuki reached out anyway, scooping up a handful and throwing it at Izuku’s face.

He dodged it, thankfully, but Katsuki cackled all the same, his hands unphased by the steaming foam.

Any complaints the teacher might have had were cut short by an explosion from behind them, Kaminari squawking as he jumped back. Somehow, beyond Izuku’s comprehension, Kaminari’s table was on fire, the foam going up in flames as quickly as it had shot from the flask.

While everyone else was leaping backward, Katsuki was surging forward, ignoring Izuku’s yelps and flailing to try to hold him back. He grabbed a fire extinguisher from the wall, shoving Kaminari away to get to the mess, a much less colourful foam bursting all over the mess when he pulled the trigger.

The sprinklers triggered before he could finish putting it out, drenching everyone and everything in the room, and with a long, tired sigh, the teacher sat down at her desk. She didn’t speak, didn’t seem to have the energy to, and after a brief pause to make sure everything was safe again, the entire class erupted with laughter. Even Iida was snorting into his hand, trying his best to cover it up, and when the bell rang no one even moved.

“I’ll help you clean,” Kaminari offered sheepishly, when things began to calm down. “Um, thanks, Bakugou.”

“Whatever,” Katsuki sighed, putting the extinguisher away on the wall. “Was hardly gonna let the building burn down, we’d never be allowed back in.”

Yaoyorozu pulled a towel out of her stomach, handing it to their defeated and exhausted teacher with an awkward pat on the shoulder.

“Why don’t you go, Sensei? We’ll get cleaned up in here, and I’ll find someone to lock up behind us. I think you could use a break.”

She walked the teacher out, and for a minute the room fell silent again, everyone staring at the destroyed room with no clue where to even start.

“Foam fight?” Ashido suggested.

The words had barely even left her lips before people began flinging, scooping up handfuls of the now-cooled foams and aiming them at each other’s heads. Izuku squawked when Katsuki turned on him again, hiding behind the table, hunting down a dry section to make his return attack.

Kaminari slipped on a puddle when he tried to move, landing face first in a pile of pink and white, Uraraka snorting when he looked up at her with a face that looked like he’d drowned in marshmallow.

The fun only stopped when they heard heavy footsteps in the corridor, looking up to see a concerned All Might staring at them from the doorway. He paused for a moment, mouth opening and closing like a goldfish as he fought for words, only choking out a confused why.

Of course, that set the giggles off again, the entire class erupting into stupid laughter, and All Might could only watch hopelessly as they rolled around in melting foam, uniforms drenched and stained with rainbows. When Iida got his composure back he quickly moved to apologise, bowing repeatedly as he assured All Might that they’d get it all cleaned up.

“Alright,” he called, when All Might left them to it. “Let’s start getting cleaned up so we can go home. Asui, Sero, Todoroki, can you please go to the locker rooms and get everyone’s gym uniforms and a bunch of towels so we can get changed? Shouji, Sato, please go find the cleaning cupboard and bring back everything you can find. Uraraka, if you can zero-gravity the tables, we can get to work cleaning them off, then move them into the corridor to clean the floors. If we get it all done without any more distractions, I’ll buy the whole class pizza for dinner.”

“Woooo!”

The promise of junk food got them all stuck in quickly, and as soon as they cleaning supplies got back, it began to move quickly. All the furniture got cleaned off and stacked in the corridor, the ingredients sorted into their rightful places in the back room, and they took turns going to clean off and change their clothes once the rest of their classmates returned. Before long, the room looked good as new, other than the drenched backpacks and textbooks that Yaoyorozu was stillworking on with a few fans and hairdryers she’d created.

By the time Aizawa showed up, there was no sign anything had even gone wrong, and Izuku smiled sheepishly when he raised an eyebrow at them.

“Sorry, Sensei!” he called, grabbing Katsuki by the arm. “We got distracted! We’re coming now!”

They excused themselves with a lot of apologies from Izuku, and Aizawa just took them to the end of the hall, eyeing them over but not bothering to question the outfit changes.

“I take it you two have made up?” he asked flatly. “No more fighting?”

“No more fighting!” Izuku agreed, beaming. “We’ve been really good!”

“We’re fine,” Katsuki agreed quietly. “It’s whatever.”

“You might even say we have a bond, now!”

Katsuki bit his lip to keep from laughing, his chest still shuddering a little all the same, but Aizawa just sighed faintly and gave them a nod.

“I’m glad you’ve... Bonded. All I care about is that you can work together, or compete against each other, without starting an all-out brawl. You’re classmates, and colleagues, even if you don’t like each other, you still have to be able to work as a team if needed. Remember that, won’t you? Not just about each other, about everyone here.”

“Understood!” Izuku nodded wildly. “Um, may we be excused? I promised to help them clean the lab, I hate having to leave them.”

“Of course,” Aizawa nodded once more. “By all means. Keep things this way, alright? Between you. It’s much more peaceful like this.”

“Absolutely!”

He headed off, back toward the teachers’ office, and Izuku breathed a sigh of relief, quickly turning into a squeak when Katsuki shoved him.

“A bond?” Katsuki huffed. “That’s the worst pun you have ever made.”

“But it’s truuuuue,” Izuku sang, linking his arm through Katsuki’s. “We bonded, Kacchan! We bonded over bad jokes and explosions, throw in some All Might and it’s just like when we were toddlers! You’re stuck with me, Kacchan!”

“Kill me.”

“Death is not a solution,Kacchan.”

“It is if it’s yours.”

“Rude.”

Despite the complaints, Katsuki didn’t pull away from Izuku’s grip, and Izuku didn’t dare to address it.

“You know,” he mumbled instead. “If you take you out of hug it gets super toxic.”

“Haah? Was that a fucking mercury joke? You’re really scraping the bottom of the barrel now, you damn nerd.”

“You understood it,” Izuku snorted. “And I’m the nerd?”

“Shut up!”

“Admit it, Kacchan, you’re just like me! We’re like... An electron pair! We uh, we don’t spin the same way, but we’re in the same place!”

“Are you saying you caught up to me?” Katsuki demanded, eyes narrowing. “Think you can beat me in a fight or some shit?”

“No! Not at all! I mean, uh... We’re in equilibrium! We’re competing, but balanced!”

“That still sounds like you think you’re as good as me.”

“I’m just trying to come up with a chemistry metaphor, Kacchan! Work with me here!”

Katsuki huffed, finally pulling his arm away when they reached the classroom door.

“You take in heat and energy, I give it out.”

“You’re exothermic and I’m endothermic?” Izuku giggled. “That sounds about right.”

“But we wouldn’t exist without each other,” he huffed, cheeks flooding with heat. “If one of us didn’t exist, the other wouldn’t need a name.”

“Yeah,” Izuku smiled. “We need each other, in our own weird way, right?”

“I didn’t say need.”

“I know what you meant,” Izuku giggled. “Thanks, Kacchan. I had a lot of fun today, we should hang out more often.”

“You mean without being forced by our homeroom teacher?”

“Exactly!”

“Yeah,” Katsuki said quietly. “I guess we can.”

“Really?!”

“Don’t act so shocked,” he grumbled, pulling Izuku’s hair and making him laugh harder. “I’ll take it back if you act like that, asshole.”

“Noooo! I’ll be good!”

They watched as everyone gathered their things, stuffing all-but-destroyed uniforms into plastic bags to either throw away or attempt to salvage, depending how many spares they each had, and when Uraraka waved, Izuku gave Katsuki one last little smile.

“Looks like I’m being called,” he said faintly. “I’ll see you later? For pizza?”

“Yeah,” Katsuki nodded. “I’m not gonna pass up free pizza.”

“And I’d better get my mattress back, too.”

“You uh, don’t have to,” Katsuki mumbled, face unusually pink again. “No school tomorrow, if you want to... hang out again.”

“Yeah! Let’s have a sleepover!”

“Sure,” Katsuki cracked a smile, nodding. “I’ll make popcorn.”

“I’ll bring all my All Might DVDs!”

“Yeah. I’ll copper later.”

“Copp- Oh my god,” Izuku snorted. “Yeah, I’ll see you later, you big nerd.”

“No one will believe you if you tell them.”

“Trust me, Kacchan, they already know.”

Katsuki watched as he walked away, feeling oddly empty considering he was literally two metres away and they’d be hanging out again in like ten minutes. After twenty-four hours practically glued to each other’s side, it already felt weird to be apart again.

“Hey, Bakubro, you coming for pizza?” Kirishima asked, slinging an arm around his shoulders. “I told Iida he has to get a veggie one, for you and your vitamin obsession.”

“Oh. Yeah. Uh, thanks?”

“Aww, Midoriya really has softened you up!” Kirishima grinned. “When are you gonna ask him out?”

“Tonight, when we sleep together again.”

He picked up his bag and walked away, ignoring the way Kirishima gaped at his back, spluttering stupid noises that didn’t come anywhere close to forming actual words. He yelped something at Kaminari and Ashido that they apparently understood, because suddenly all three of them were sprinting after him, and all he could do was groan as he prepared himself for the onslaught.

He really should ask, though. Not even a chromatograph would separate them if he had any say in it.