Work Text:
A Scientist’s Guide to Handling Hazardous Variables
by: L. Luthor
(Variable CK-01, “Clark Kent”)
Classification: Internal Use Only
Note: Not peer-reviewed (thank god).
1. Label Clearly: Not For Long-Term Use.
All hazardous materials must be labeled “Not for Long-Term Use”. Labels should be clear, concise, and adhered to at all times.
Casual. Temporary. Non-committal. That’s the label he slapped on Clark Kent the moment this arrangement began. Fuck buddies, or whatever euphemism people use these days to dress up the concept of mutually satisfying but ultimately meaningless encounters. Lex had been firm about it, or at least he told himself he had.
It was supposed to be a way to de-stress. The board, the investors, the endless parade of meetings and expectations—there was no better release valve than Clark Kent showing up at ungodly hours, all wide-eyed earnestness and infuriating strength, and pinning him against a wall. He would never admit it aloud, but the man knew what he was doing. Thoroughly. Disarmingly. Enough to make him bite down on the inside of his cheek to keep from making sounds he’d regret later.
And that was the problem. Clark was good, too good. He hated the idea that he might never find someone else who could fuck him senseless the way Clark Kent could. It made him feel dependent. That was the kind of vulnerability Lex Luthor didn’t tolerate, not in his professional life, and certainly not in this so-called casual arrangement.
“This is just sex,” he told Clark, leaning back against his headboard with all the detachment of a man who hadn’t just been fuck stupid just a few minutes ago.
Clark, maddeningly, had only grinned. “Sure. Just sex.”
Too agreeable. Suspiciously agreeable. Lex should’ve known then.
The problem is, Clark Kent ignores labels. He files stories late, misses deadlines, leaves his suit jacket draped over Lex’s couch like it lives there now, and apparently disregards emotional warning stickers, too.
It was supposed to be a casual arrangement. Something neat and compartmentalized. A billionaire and an alien farmboy meeting in secret to “de-stress.” He gets the satisfaction of being ruined in ways no one else could ever manage. Clark gets… well, Lex isn’t sure what Clark gets, other than opportunities to drive him insane.
That was the theory.
The practice looks more like this: Clark showing up at his penthouse at midnight holding a pink box from Dough’s Hole.
Lex opened the door, saw the familiar packaging, and pinched the bridge of his nose. “You cannot possibly think fried dough qualifies as foreplay.”
Clark just gave him an infuriating smile, dimples on display. “Didn’t say it was foreplay. Just thought you might want your favorite. Got the cinnamon twists you like.”
Unacceptable. Clark Kent was not supposed to know his favorite anything. He had only mentioned Dough’s Hole once, in passing, while ranting about an incompetent assistant. Yet here was Clark, remembering. Worse, repeating the offense regularly.
That is not casual.
Then there’s the cocoa. The first time Clark bypassed Lex’s imported French roast beans and went straight for the hot cocoa mix—cheap, powdered, probably made of 90% sugar and 10% cocoa, he nearly had him arrested. But now, somehow, there’s always a stash of cocoa in his pantry. He doesn’t even remember buying it anymore. It just appears, like Clark has annexed a section of his kitchen.
Last week, Lex caught himself automatically filling two mugs: one coffee, one cocoa. He shoved the second across the counter without thinking.
Clark took it, smiling like Lex had just handed him the keys to the city. “Thanks. You always make it better than I do.”
“I literally just boiled water.”
“Exactly,” Clark said, dead serious, and he had to turn away before Clark noticed he was smiling back.
That is definitely not casual.
Lex doesn’t make space for people. He doesn’t even stock food for himself. But now there’s a box of Clark’s cocoa mix tucked beside the teas, and sometimes he swears the man leaves his laugh behind in the penthouse when he goes. Lex has faced down senators, CEOs, even Batman, but none of them terrifies him the way the sound of Clark’s easy laughter does in his own living room.
Casual, he reminds himself. Temporary. Non-committal. He repeats it like a prayer every time Clark leans against his counter, sipping hot cocoa, looking at him like there’s no place in the world he’d rather be. He really shouldn't get used to this. It will end eventually.
The label is there. It’s written in all caps across the file folder in Lex’s mind.
Clark, of course, ignores it.
One evening, when he muttered something about “temporary arrangement” under his breath, Clark had the audacity to tilt his head and say, “You keep saying that like you’re trying to convince yourself.”
Lex had glared, sharp as a blade. “I’m reminding you.”
Clark only smiled. “Right. Sure, Lex.” Fucking dimples.
And Lex, for once in his life, didn’t have a counterargument.
2. Handle With Care.
Use protective measures at all times (emotional detachment). Avoid unnecessary exposure (don’t get TOO involved with Clark’s life).
He really should be careful. When they started fooling around, he knew this thing they had would have to end sooner or later. Clark is Superman. He belongs to the world, not to Lex. Which is fine, he tells himself. Perfectly fine. He wasn’t asking for forever. He wasn’t even asking for a season.
So he really shouldn’t imagine a future with Clark in it.
Clark in his kitchen. Clark leaning against the counter in his shirtsleeves, smiling like he actually lives there. Clark insisting on breakfast for dinner because “pancakes taste better after sunset, Lex, you’ll see.” Clark sliding into his life like he was made for it.
Lex knows better. He does. But sometimes it’s hard not to picture it. Harder still to remind himself that this isn’t a shared life, just a series of nights stitched together with takeout and stolen kisses.
The lines were already blurry, but they began to smear altogether the day Lex, god help him, met Clark’s parents.
It wasn’t supposed to happen. Of course it wasn’t. Lex doesn’t do parents. He doesn’t do family dinners. He barely does boardroom lunches. But Clark had asked him to tag along on some grocery run. A grocery run, of all things.
Lex had scoffed at first. “Clark, I have people who do that for me. I haven’t set foot in a supermarket since I was fifteen.”
Clark had only given him that look, that mixture of amusement and stubbornness that somehow always manages to win. “Come on. It’s not that scary. You could use a normal Saturday for once.”
And, because Lex is apparently weak when it comes to men with impossible ocean eyes and irresistible dimples, he agreed.
Which is how Lex Luthor, CEO of LuthorCorp, ended up standing in the fluorescent glow of a budget supermarket aisle, debating the merits of two nearly identical brands of laundry detergent with Superman. Lex held one bottle in each hand, scowling. “They’re the same.”
“They’re not,” Clark insisted. “One’s eco-friendly.”
“Eco-friendly,” Lex repeated flatly. “You fly across the world, Clark. I think your carbon footprint has already—”
“Shh,” Clark cut in, grinning. “Pick the green one.”
Lex muttered something about moral blackmail but tossed the eco-friendly bottle into the cart anyway. Clark kissed his cheek right there between the detergent and dish soap, and he had to pretend his ears weren’t burning red.
That should’ve been the end of the day’s indignities. Instead, after groceries, Clark suggested they swing by his apartment. Fine. Whatever. Lex tolerated it. But fate, or irony, or some particularly cruel god decided to intervene.
Clark’s parents showed up.
Unannounced. From Kansas. At his apartment.
Lex had frozen in the doorway, caught like a deer in headlights as Martha Kent, apron in her handbag, because of course she had one, beamed at him like he was just another guest and not the man currently sleeping with her son. Jonathan shook his hand, all farmer-calluses and firm grip, and Lex was certain he’d just failed a test he hadn’t studied for.
Clark, to his credit, looked mortified. “ Ma! Pa! This is uh-”
“Lex!” Martha finished brightly. “Oh, we’ve heard about a lot of things about you. Come on, sit down! You’ll stay for dinner, won’t you?”
He should’ve declined. He meant to decline. He even opened his mouth to say something cutting and polite. But then Martha set down a still-warm plate of eggs, bacon strips, sausages, and pancakes. Wow, they really do love breakfast for dinner, huh.
Lex, traitor to himself that he is, sat down at the table like he belonged there.
It didn’t help that Martha Kent made an excellent apple pie. Possibly the best thing he’d ever tasted. Certainly the most dangerous.
Somehow, between bites, Martha asked for his number. And somehow, because she smiled and smelled faintly of flour and kindness, he gave it to her. Now Clark’s mother has his personal contact information and occasionally texts him invitations to their home in Smallville. To Smallville, Kansas. Where Clark grew up. Clark’s childhood home.
Lex Luthor doesn’t get invited places. Not homes, anyway. Not by mothers.
And yet, here he was, standing in a cheap supermarket one day, and the next, sipping coffee at Clark’s apartment while Martha Kent pressed a second slice of pie onto his plate. Like it was the most natural thing in the world. Like he belonged there.
That was the problem.
Because he doesn’t. Not really. He doesn’t belong in kitchens with cocoa packets or in supermarkets debating detergent, or in Clark’s childhood home eating pie that his mother made. He doesn’t belong to that world—or to Clark.
And yet, when Clark leaned over later, brushing his shoulder in that casual way that wasn’t casual at all, Lex had the sudden, dangerous thought that maybe he wanted to.
Which was, of course, the most hazardous thought of all.
3. Do Not Store Overnight.
Hazardous materials must never be stored overnight.
The rules are simple. Clark comes over, they fuck, and then he leaves. Simple.
The reality, however, has been…complicated.
Clark shows up with snacks, complains about his movie choices, and then, without fail, falls asleep halfway through.
“You can’t possibly be bored,” Lex mutters one night, watching Clark’s eyelids droop as the villain monologues on screen.
“Not bored,” Clark says, voice already soft and drowsy. He yawns. “Comfortable.”
Lex raises a brow. “Comfortable? That’s your excuse?”
Clark tilts his head until it lands, quite heavily, on Lex’s shoulder. “Mm. You’re warm.”
Lex blinks at him. He has personally been called many things in his life: brilliant, ruthless, terrifying, unfeeling. But warm? That one feels like a personal insult.
And yet, instead of shoving him off, Lex lets him stay. Watches the way Clark’s breath evens out. Watches his lashes flutter against his cheek. Watches, and wonders what it would be like to wake up with this every morning.
He hates himself for the thought.
One morning, Clark’s in a rush. He pulls on yesterday’s shirt, mismatches his socks, mutters about being late for a deadline, going back to his apartment because of course, he doesn’t keep spare clothes at Lex’s penthouse. Why would he? This isn’t a relationship. This is casual. Temporary. Non-committal.
And yet, watching him scramble, Lex’s brain betrays him. It would be easier if he lived here, he thinks. Move in with me.
The thought lands like a slap. A toothbrush beside his. A second coffee mug left in the sink. Clark’s laugh echoing through the halls. Too much. Far too much.
The rule is clear: don’t let him stay the night.
But rules, apparently, mean nothing when Clark Kent is involved.
The first time Clark stayed, it was an accident. He fell asleep on the couch.
The second time, he migrated to Lex’s bed. “Your sheets are ridiculous,” Clark mumbled, burying his face into Egyptian cotton. “You could sell tickets just to nap in here.”
The third time…Lex didn’t ask him to leave.
“Don’t get used to this,” he said, lying very still while Clark sprawled beside him like he owned the place.
Clark cracked one eye open, already grinning. “Sure. Absolutely. Not getting used to it at all.”
He stayed until morning.
And the night after that.
And the night after that.
It keeps happening. Clark keeps staying. And every time, Lex tells himself it’s the last. Every time, he tells himself not to imagine futures he can’t have: Clark’s shoes by the door, Clark’s tie draped over his chair, Clark’s clothes lined up neatly beside his.
And every time, he fails.
Because the truth is, Lex doesn’t just let him stay the night. He wants him to. Desperately.
4. Keep Distance Whenever Possible.
Maintain a safe distance between the scientist and the subject to prevent entanglement.
“Casual” requires boundaries. And Lex knows boundaries. He built an empire on them. But lately, those boundaries with Clark are blurring, and it’s starting to mess him up. So he does the only logical thing he knows: he cuts Clark off. No calls, no texts, no late-night visits.
Cold turkey.
It’s harder than he expects. He’s gotten used to the presence of a six-foot-four, dimpled alien who knows his body better than anyone else ever has, who worships him like a religion. Lex, for the first time in his life, actually considers celibacy.
For a month, he keeps his distance. For a month, he thinks he’s winning.
And then fate laughs in his face.
The gala is in Gotham, black-tie, full of people Lex doesn’t like but has to pretend he tolerates. He’s doing fine, drinking, glaring, existing, until he sees him.
Clark Kent. Velvet suit. Red bowtie. Looking infuriatingly good.
Lex immediately considers setting the entire venue on fire.
“Wasn’t expecting to see you here, Mr. Luthor,” Clark says when they finally cross paths. The smile doesn’t reach his eyes. His tone is polite. Too polite.
Great. They’re back to this. Back to Mr. Luthor. Back to looking at each other like they’re strangers, not men who’ve spent nights tangled up in sheets, breathing each other’s names.
“Sure you weren’t,” Lex replies coolly, sipping his drink.
Clark opens his mouth to retort, but then—
“Smallville! There you are!”
They both turn. Lois Lane strides over, bright and confident, and Clark’s entire face changes. His gaze softens instantly. His hand slides around her waist like muscle memory.
Lex felt a pang in his chest.
Lois leans in, whispers something in Clark’s ear, and Clark smiles. Softly. The kind of smile Lex thought was just for him.
“Oh! Mr. Luthor!” Lois greets cheerfully. “Great seeing you here!”
“Likewise, Ms. Lane.” Lex’s voice is smooth as glass, but the taste in his mouth is bitter, metallic.
He doesn’t wait another second. He calls Mercy. They’re leaving Gotham tonight. If he stays any longer, he might do something he regrets.
Hours later, back in Metropolis, there’s a knock at his door.
Clark.
Lex leans in the doorway, arms folded, keeping his voice even. “To what do I owe this midnight visit?”
“You know,” Clark says, stepping inside anyway, “you’re a bit unfair to me.”
“Unfair?”
“A month of silence,” Clark says, exasperated. “Nothing. No calls, no texts. I thought maybe you were done with me. I tried to respect it. But then I see you tonight and you look at me like I’m the one who did something wrong.”
“I don’t know what you mean.” Lex’s voice is sharp. “And why are you mad? I see you got yourself a girlfriend.”
Clark stares at him. “Lois is my best friend.”
“Really? That’s not how it looked.”
“Lex—” Clark drags a hand through his hair, frustrated, but when he looks back his voice is quieter. Softer. “Look. This arrangement might be nothing for you, I know that. But isn’t it unfair that you’re the only one who gets to call the shots? You decide when I see you. You decide when I leave. You decide when to disappear. And I just let you.”
Lex’s throat tightens. He hates how the words land, too true, too sharp. He lifts his chin, schooling his face into something cool and dismissive. “Why are you so worked up? We said this would be casual.”
Clark lets out a short, bitter laugh. “Casual. Right. Easy for you to say.”
“That’s what you agreed to,” Lex snaps back, faster, harsher than intended. The edge in his voice is a shield.
Clark shakes his head, gaze locked on him, too earnest, too unguarded. “I thought I could handle it. I thought I could keep it simple, like you. But every time you look at me—” He stops himself, biting the words back. His voice roughens. “Damn it, Lex, is it wrong to hope that I’m not the only one who wants more?”
And there it is. The thing Lex has been dreading and craving in equal measure.
Lex freezes. The words burn through him, dangerous, impossible. For a moment, he lets himself imagine it. Clark in his kitchen every morning, not just sometimes. Clark’s hand brushing his, always. Clark choosing him, despite everything. The image feels like sunlight, like something he has no right to touch.
“I don’t know what you mean,” Lex says finally. His voice is sharp, cold, but it trembles at the edges.
Clark steps closer, heat radiating off him, ocean eyes burning with hurt and stubborn hope. “I think you do. You were keeping your distance, Lex. You’re pretending that this is nothing. And I’m the one standing here, wanting more, wanting you, and you won’t even admit it.”
His chest feels too tight, like he’s standing on the edge of a cliff. Every instinct screams at him to reach out, to close the gap, to say yes. But he can’t.
Clark is Superman. He is light, kindness, and hope personified. He’s the boy from Kansas who wears his heart on his sleeve.
And Lex—Lex is Alexander fucking Luthor. Difficult. Cruel in more ways than he can count. A man who built walls of steel around his heart and then pretended to be surprised when no one got through.
But Clark did. Somehow, impossibly, he slipped past the defenses, bright and stubborn and unrelenting.
And Lex hates him for it almost as much as he loves him.
5. Dispose After Use.
That was the plan. That was always the plan.
Casual. Temporary. Non-committal.
Lex was very clear about it, at least to himself. You don’t keep Clark Kent. You don’t invite permanence when permanence is the one thing you can never trust yourself with. The rule was simple: when it’s over, it’s over. No messy attachments. No longing. No danger.
Except Clark Kent refuses to be disposed of.
Clark is standing in his penthouse door again, a box of doughnuts in hand like a peace offering, like he belongs here. And Lex is tired—tired of running, tired of lying to himself, tired of pretending that this doesn’t feel terrifyingly close to home.
“You shouldn’t be here,” Lex says, but the words come out thinner than he means, almost pleading.
Clark just shrugs, unfazed, and holds up the pink box like it’s armor. “Brought cinnamon twists. Don’t pretend you don’t want one.”
Lex narrows his eyes, but his body betrays him, stepping aside. “You’re insufferable.”
Clark breezes in, already toeing off his shoes like he owns the place. “You let me in, so what does that make you?”
“An idiot,” Lex mutters.
“Maybe.” Clark sets the box on the counter, then looks at him, really looks, like he can see through every wall Lex has spent years perfecting. “But I think it makes you something else, too.”
Lex hates how his throat closes. “Don’t.”
“Don’t what?” Clark’s voice softens, steady and unbearable. “Don’t say what’s true? That you care? That this isn’t casual for you any more than it is for me?”
“Fine. You want honesty? You want the great reveal? Here it is. I want you here. Every night. Every morning. Every mundane errand in between. I want things I shouldn’t. I want to keep you, Clark. And it terrifies me, because you’re you” he gestures at him like it’s self-evident, like the word is too dangerous to say aloud, “And I’m this. Sharp edges and bad decisions in a thousand-dollar suit. I’ll ruin it. I’ll ruin you. That’s what I do.”
The words sit there, ugly and bare. Lex braces for Clark to walk out, to prove him right.
But Clark just… laughs. That quiet, disbelieving laugh that makes Lex want to throw him off the balcony and kiss him senseless in equal measure.
“You really don’t get it, do you?” Clark says, stepping forward, setting the pink box of doughnuts on the counter like this is the most ordinary night in the world. “Lex, I never wanted casual. Not once.”
Lex blinks at him. “Excuse me?”
“From the start,” Clark says, steady, infuriatingly earnest. “You said ‘just sex.’ I agreed because I knew if I argued, you’d bolt. So I took what I could get. And I kept hoping you’d figure it out eventually.”
Lex stares at him, stunned. All those rules, mantras, and compartments, and Clark had been quietly dismantling them all along. Cocoa in the pantry. His suit jacket in the living room. Socks abandoned at the edge of the bed. Not accidents. Not oversights. Intentional.
“Oh my god,” Lex breathes, half to himself. “You’ve been domesticating me.”
Clark’s mouth quirks. “Worked, didn’t it?”
Lex throws his hands up, exasperated. “God, I’m an idiot.”
Clark grins, dimples in full force. “Maybe. But you’re my idiot.”
Lex glares at him, because it’s the only defense he has left, but his chest feels too tight, too full. All the rules, all the labels—burned to ash by something as stupidly simple as Clark Kent standing in his kitchen with doughnuts and the audacity to love him.
He doesn’t have the strength to fight anymore.
Clark steps closer, slow, deliberate, until Lex can feel the warmth radiating off him. His hand comes up, cradling Lex’s jaw like he’s something fragile, something worth keeping safe.
Lex should push him away. He doesn’t.
Clark kisses him. It starts careful, tentative, like Clark is giving Lex an out. He doesn’t take it. He fists a hand in Clark’s tie, pulls him closer, and deepens the kiss until Clark makes a sound so reverent it nearly undoes him.
The kiss turns hungrier, less patient. Clark presses him gently back against the counter, and Lex lets him, reveling in the absurdity of being worshipped by a god. He breaks the kiss only to gasp against Clark’s mouth, “You’re insufferable.”
“Good thing you like insufferable,” Clark teases, tugging him closer.
Dispose after use? Please. He’s never letting this go.
Two weeks later, Lex found himself in a situation he would have once considered catastrophic.
There was a toothbrush beside his.
Clark’s. Blue handle, cheap drugstore brand, offensively cheerful against his black marble sink.
There was also a mug, bright red, with a faded Daily Planet logo, sitting in his pristine glass cabinet. Next to Lex’s immaculate crystal tumblers, it looked like vandalism.
And yet, somehow, Lex hadn’t thrown either of them out.
Even worse: he’d caught himself smiling at the sight.
He should’ve been horrified. He was horrified. But then Clark wandered out of the bedroom, hair a mess, shirt half-buttoned, and grinned like Lex was the best thing he’d ever seen, and Lex decided maybe this wasn’t so bad after all.