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It happens as an accident. Steve catches Tony one morning, innocently intending to grab his boyfriend by the waist and draw him in for a hug. But the reaction he gets when his fingers poke Tony's side is different from what he expected.
Tony cringes away with a sound that's half a gasp, half a choked giggle, before he whirls around to glare at Steve – Steve, who is stunned by the noise he's just heard.
“Don't”, Tony growls, when Steve's smile grows and grows until it threatens to split his face. “Don't you dare. Steve Grant Rogers, I swear-”
Steve pulls him close and kisses him.
Of course that's not the end.
Steve is aware he made a very precious discovery. Tony is ticklish, at least right there above his hip and Steve can't help but wonder if his lover has any other ticklish spots.
So he makes it his personal mission to find it out.
The next time he tries something is when they are cuddled up on the couch. Tony's head rests on his shoulder and Steve feels a wave of fondness wash over him whenever he gazes at the relaxed, peaceful expression his boyfriend sports. Tony looks thoroughly comfortable.
Steve's got his arm around him, tightening just a little.
Tony hums and presses closer, not expecting anything, so Steve moves his fingers a little and probes at Tony's ribs.
The reaction comes instantly. Tony gasps and, when Steve doesn't stop but pokes him again, lets out a startled laugh.
Soon enough, Steve has Tony curled into a ball on the couch, arms wrapped around his chest in a vain attempt to protect himself. And he is laughing, giggling, really. Steve is mesmerized by the sound; it's so beautiful and he falls in love all over again at the way Tony's face lights up as he laughs.
“St- Steve- Steve!”, his lover gasps, breathless. “S-Stop it-”
Steve does pause, grinning at Tony, who glares at him. His cheeks are flushed and his eyes, warm and brown, sparkle with hidden amusement.
“Asshole”, Tony grumbles, but his lips curve up. Steve kisses the smile right off, hands now moving up to caress Tony's shoulders and tangle in his hair.
“You never told me you were ticklish”, Steve mumbles between kisses. He goes down willingly when Tony pulls him, careful not to crush his boyfriend with his weight.
“'s because you'd abuse this knowledge”, Tony accuses. He raises an eyebrow at Steve as if to dare him to argue, but Steve only ducks his head with a rueful expression.
The blond steals another kiss. “It makes you laugh”, he defends himself. “And I love it when you laugh.”
Tony gives an theatrical groan, pretending to be annoyed.
“Ugh, there you go, being all sappy.” His boyfriend pushes him off and gets up from the couch, stretching. It's just enough to make his shirt ride up, to expose a patch of olive skin and-
Tony squeaks when Steve grabs him once more and tickles him, his giggles soon dissolving in a string of curses.
“Steven Grant-”
He doesn't get further than this, cut off by Steve's fingers probing at his stomach next.
They end up chasing each other all the way to the bedroom, where Steve traps Tony on the bed, with mischief sparkling in his blue eyes.
Tony resigns to his fate with a sigh before he pulls Steve in for a long kiss.