Tony thinks he’s pretty good at noticing things about Bruce. He’s memorized the way he presses his glasses into the bridge of his nose when he’s frustrated, how the corners of his eyes crinkle up during his rare smiles, the exact arrangement that his curls fall in over his forehead - everything he loves about his boyfriend. So it doesn’t take him long to realize that Bruce likes his arc reactor. He first notices Bruce gazing at the circle of light glowing through Tony’s shirt in a sort of dreamy way when they’re working together and soon enough Bruce asks to examine the thing, which results in them making out for the first time against one of the lab tables. From then on, it’s been a thing for them. Whether they’re working in the lab or having sex, Bruce always pays special attention to it, and it’s honestly a major turn-on for Tony. Bruce will touch the reactor whenever he can get close enough to get his hands on Tony, pressing his palm against its flat surface or curling his fingers around its protruding edge. It seems almost territorial the way Bruce stakes his claim on Tony’s most personal piece of technology, as if by entering into a relationship with Tony he came to own a part of it, a part of him. But Tony still doesn’t know why Bruce is so attracted to the device in his chest, and he’s desperate to find out.
So one night while they’re sitting in bed together, leaning against stacked pillows and quietly doing their own separate things, Tony decides to ask him, throwing the question at him out of the blue as he usually does.
“Why do you like my arc reactor?”
At Tony’s inquiry Bruce glances up from the book he’s reading and quirks an eyebrow. He’s wearing a plain t-shirt, pajama pants, and his glasses. The bedcovers are nestled around his body to keep him warm in the cool bedroom, and a reading lamp is perched on the nightstand beside him.
“Why do I like it? That’s a strange question, Tony.”
“Yeah, you know. I mean, it makes a good night light, but you seem to think it’s... more,” Tony says with a slight shrug, lightly tossing aside the phone he’d been using to play Angry Birds while Bruce read and giving Bruce his full attention.
“More than just a night light?” Bruce chuckles, dog-earing the page he’s on before setting the book next to the lamp. He looks over at Tony, crossing his arms over his chest. “Of course it is. It’s a scientific masterpiece - a remarkable piece of machinery, by far the most incredible I’ve ever seen.” Now his own words seem to fill him with awe, and he unfolds his arms to look into his open hands as he shakes his head almost imperceptibly. “It’s crazy enough that it’s able to power an electromagnet on its own with purely clean energy, and coupled with the fact that it’s actually acting inside your body... It’s amazing, Tony.”
Tony studies Bruce’s face as he finishes and crosses his arms again, and Bruce meets his gaze curiously through his glasses. The answer is good - Tony will never get tired of hearing Bruce talk in that brilliant scientist voice of his - but he knows there’s more lurking underneath. Bruce just needs a little more prodding to let it all out.
“Yeah, the science is great and all,” says Tony, “But why do you really like my arc reactor?”
Bruce narrows his eyes slightly, and then his lips form a knowing smile and he removes his glasses with one hand, setting them down atop the book and switching off the reading lamp with a small click . He slowly lowers his gaze from Tony’s to settle on the arc reactor, which is glowing bright in Tony’s bare chest. Now that the lamp is off, it is the only light source in the dark bedroom. Its blue hue bathes over Bruce’s features and reflects in his eyes, twinkling there like stars.
“You want to know why I like your arc reactor, Tony?” Bruce replies softly, shifting closer to Tony, who just grins at him. It’s the kind of grin he gets on his face when he knows he’s about to get exactly what he wants.
“There are a lot of reasons,” Bruce says, and he’s now close enough to reach over and gently place his hand over the exposed reactor, eclipsing its glow and making himself all but disappear. Tony can’t help but press his body up into Bruce’s hand, enjoying the slight pressure it put on the metal embedded in his chest.
“I like how smooth it is,” Bruce begins, and Tony has a feeling this is going to turn into a list of some sort and he can’t wait to hear the rest. Bruce easily slides his hand off the reactor as if to prove his statement, returning light to the bedroom and illuminating Tony’s wide grin.
“I like how bright it is,” Bruce continues in a slightly softer voice, and now he raises his hand up to dance his fingers in the blue glow, throwing shadows on the ceiling. Tony watches as Bruce’s eyelids droop ever-so-slightly, and he prompts him, unable to keep the absolute glee out of his voice, “What else?”
And Bruce returns Tony’s devious grin, now, and moves over him to place a knee on either side of Tony’s hips, essentially straddling his legs. Tony looks up at him and the light of the reactor that Bruce loves so much is causing a shadow to form on his brow that, coupled with that grin, makes Tony’s stomach do a somersault. Bruce leans forward to put his hand over the arc reactor again, fingers spread out this time so that the light shines through, illuminates the veins beneath the skin of his fingers in a clash of blue and red.
“I like how it’s always slightly warmer than the rest of your body,” Bruce murmurs, his voice getting softer and huskier with each addition to the hopefully long list of reasons why the arc reactor is fantastic and he should keep touching it like this forever because it’s making Tony warmer in a lot of other places as well. “I like how, if you listen closely, you can hear it humming -”
“Keep going,” Tony half-chokes on the words, his heart rate accelerating beneath Bruce’s fingers. Bruce presses a little harder into the reactor, his eyes focused on Tony’s, wide and attentive and absorbing and, fuck , he knows exactly what he’s doing to Tony right now.
“I like how, when you get excited like this - I can feel your heartbeat vibrating through it -”
Tony makes kind of a strangled gasping sound as Bruce pushes even harder and Tony knows what he’s feeling because he can feel it too, the pounding that shakes the entire arc reactor from its deepest core to the edge of Bruce’s skin. Tony brought this upon himself and he knows it, and his already stupidly huge grin grows impossibly wider as he feels Bruce placing both hands on his chest, one on either side of the reactor, and slowly dragging his thumbs around the edges of it. Bruce leans in closer, hovering just above Tony’s face.
“I like -“
He is cut off as Tony tangles his hands in Bruce hair and drags him forward into a kiss, unable to wait any longer, and Bruce willingly reciprocates it, open-mouthed and eager. When Bruce gets a chance to break away, he continues breathlessly, “ - I like how it’s not completely embedded in you, I can - ”
Again Tony can’t wait for the rest and he pulls Bruce’s stupidly beautiful lips into another feverish kiss, which breaks after a moment as Bruce begins to trail kisses down his chin and throat and clavicle and sternum, ending in a kiss to the center of the arc reactor.
“Fuck , Bruce,” Tony moans as Bruce meets his eyes, head still hovering inches from the arc reactor, illuminated by its glow. His face is flushed and his hair is messy from Tony’s fingers curling in it, but he still isn’t finished.
“I like how sensitive you are around it - ”
Bruce cuts himself off to run his tongue around the edges of the reactor, dragging across both smooth metal and the skin surrounding it and his fingers trace over the ghosts of scars and Tony just moans and tightens his hold on Bruce’s hair as fireworks explode in his brain. He slams his head back against the headboard and arches his spine, and Bruce presses his lips to the arc reactor’s glowing core once more before covering it with his hands again and moving back up, pausing to kiss the throbbing pulse in Tony’s exposed throat before he meets his lips. Tony just loses himself in the moment, forgetting all about the question and the answer and all it is is him and Bruce and Bruce’s hands on the reactor that is so many amazing things and yet nowhere near as amazing as the feeling that’s currently turning Tony’s insides to mush.
When Bruce finally pulls away to breathe and calm his racing pulse, Tony sinks back into the pillows, also panting, and releases his hold on Bruce’s hair. His hands travel down Bruce’s chest and then back up beneath his shirt, cupping the spot, where, if they were to switch places, Bruce would have an arc reactor of his own. Bruce closes his eyes, smiling at Tony’s touch, and whispers, “There’s something I like more than anything I’ve told you so far.”
“What is it?” Tony asks, his breath hitching in his throat as Bruce gazes down at the light that shines between them. His fingers caress the reactor’s bright surface with such a delicate gentleness that it’s almost impossible to believe that hidden within his blood is a creature powerful enough to crush it into a million pieces and snuff out Tony’s life instantly. The pause lasts twenty heartbeats felt through metal and when Bruce finally speaks it’s in a voice heavy with emotion.
“The reason I like the arc reactor, Tony,” Bruce finally reveals, and Tony realizes that he knew it, deep down, all along, “Is that it’s keeping you alive.”