The way she smiles

Clint

Clint was the one who noticed first (Well he always does). After spending years as her partner, he was the one who got notice her changes ever since he brought her to S.H.I.E.L.D.

And now as he look at her (not in a creepy way though) he can see there's another change in Natasha Romanoff.

Clint became aware of how all of the sudden, she would join the antics of the boys. He also thought he saw a sparkle on her eyes everytime she laughs (which was a rare one). And of course, who could forget her smile. Clint is one of the lucky ones who got to see her real smile, and now, she's practically showing it off to everybody.

He was curious, he's in the verge of asking her infront of the Avengers. But knowing her, she would still want to talk about this subject in private, because she's Natasha Romanoff

"Clint!" His thoughts were cut by a female voice, the voice of the woman consuming his thoughts

Clint looked up at her "What's wrong?" Because obviously he just woke up from his thoughts

Natasha Romanoff knows Clint Barton too well, and whenever he always zoned out, she would always get the answer "You zoned out" she said standing up "And I want to know why"

"It's nothing" he replied and before he could add another reason why she shouldn't talk to him, he was pulled out of the common room and been brought to the balcony

"Okay Barton, talk" She ordered with her dominant voice yet he could still see softness in her eyes

"Well, I would want to know why my partner slash my best friend why all of the sudden, began to smile alot" Clint answered without any hesitation and with that question, she just smiled at him.

"There's someone huh?" He asked

"What do you mean?" She replied

He looked into her eyes "Tash, you know I love you as a sister. We've been partners for years. And you're my only bestfriend, well besides the other guys but if there would be a list, you would be number one." He said "And you know me that for every single change you have, physically and emotionally, I would always be the first one to notice"

Natasha just smiled at him (which was the most sweetest thing he ever saw) and replied "Yeah, there's someone"

Tony

Tony realized that he's been threatened by Natashalie (his nickname to her) only once a day, and it was a world record to be threatend by Natasha Romanoff once a day. Normally, she would threat him by brekafast, lunch and dinner, there were also threats from her during their meetings, their practices and basically, everytime she sees him.

And Tony knows that there is something behind little red riding hood's (another nickname) smile.

His jaw literally fell when he first saw her smile. It wasn't like the casual smirk when she would win their arguments, or that grin that she would send him everytime she would beat the crap out of him during practice. IT WAS A LEGIT SMILE

And Tony Stark, being Tony Stark aka being the Queen of the Gossip (as Natasha would call him) plucked up the couraged and ask her one simple question

"Hey red" he called as he entered the gym for the team "I am pretty sure that the dummy is dead"

"Of course he's dead, it's an object" she replied, without the cold glares or the toughness in her voice, it seemed like the literal normal (which was a first)

"You know what I mean" he said "Hey red, can I ask you a question?"

Natasha stopped punching the dummy "It depends if the question is worthy of being answered"