They have a cat.
Which is weird since none of them are particularly cat people.
Bruce sees it first while doing yoga in one of the rec rooms. The cat, no longer a kitten but not quite full grown either, is watching him from under the low couch that he keeping water bottles on. It's got a jet black coat with white mitten paws and blue eyes. Bruce blinks at it, still upside down. He chooses to ignore it for the time being, focusing on moving through the rest of his positions.
Once he's done though, he glances back towards where the cat is. When it notices him watching it, it yawns and slinks out from underneath the small space, spine bending in a way only a cat's can. Bruce runs his thumbs over each of his fingers before leaning down and sticking one of his hands out. He's not really expecting much, most animals tend to be very (reasonably) weary of the monster inside him.
The cat comes closer and sniffs at the offered fingers. It slowly rubs each cheek over his fingers and then mews up at him.
Bruce isn't a cat person, but if he chooses to keep bits of string on him from then on, no one needs to know.
Clint wakes up from a nightmare on the couch, the little cat curled up against his stomach, purring up a storm. It licks at the skin that's been exposed by his shirt ridding up and then purrs some more. Clint snorts and gives in to affection that it's offering (and eventually begging for. It follows him into the kitchen and then into the fucking bathroom so that it's not going without warm Clinty ear scratches for more than 30 seconds once he gets started).
The cat sleeps at Natasha's side as she sharpens knives. She teaches it how to play fetch and get up to the alarm notification button in case there's an emergency. She has fun dressing it up in little sweaters and hats. She forwards her Catvengers collection of pics to Pepper.
Steve first sees it on top of the fridge in the common area. He is…more than a little confused as to how it got in, especially considering Tony's no pets rule, but figures he either must have changed his mind or simply doesn't know about its presence. Steve's not about to question that one way or the other. He actually is a cat person.
He offers up a hand to it and gets several licks, to his delight. He is more than a little giddy by the whole thing, often holding the kitten in his arms or buying toys for it. So many toys. It's not really even his cat though so he (barely) refrains from buying this one cute collar with little mouse shaped chimes on it. It's possible but unconfirmed that he has a couple of pictures of the cat on his phone. (It's confirmed. By Sam. Several times a day. Sam cringes at the thought of what Steve might be like should he have actual children.)
Thor takes the cat hunting. The others…are not entirely pleased with this.
"What the fuck is that?" Tony asks. The group freezes guiltily to look up at him. The cat is on the kitchen counter, licking the remains of Clint's freshly made bacon from it's paws.
"It's…a cat?" Steve offered slowly.
"Who brought a cat into my building?"
The superheroes glance at each other, none looking more guilty than another. Clint frowns slightly.
"Wait…so…none of you guys brought it in either?" he asked.
"No. No. We're in a highly secure building. Flies can't get in without someone knowing. How did a CAT get in here?" Tony said.
When he didn't receive an answer, Tony turned his attention up.
"Jarvis, who brought this thing in here?"
"No one did, sir."
Tony squinted up towards the ceiling and scowled. He stalked over to the cat and picked it up under the arms, getting a soft irritated mew in response. He brought the cat up to eye level and held it close, glaring.
"Where did you come fr-" he froze and leaned back slowly. He leaned in and then pulled back again. "Huh," he muttered.
Steve moved to stand just behind Tony. "What? What is it?"
Tony put the cat down and smiled wide. He cupped its face and started stroking its whiskers with his thumbs. "Look at you! Look at you! Aren't you just the cutest thing! And your fur, fucking hell." he breathed out, with a sort of helpless delight.
His teammates stared at the billionaire with alarm. Maybe the cat had mind control powers? To…get people to take care of it?
Bruce carefully placed a hand on Tony's shoulder. "Tony…" he said.
The man's head suddenly shot up. "Its eyes are blue. Not just any blue. Arc Reactor blue." he said, looking up at them with pride.
They frowned, confused, before realization hit.
"Jarvis…did you…did you make us a cat?" Clint asked.
"I have found that owning a pet increases one's life expectancy. And, in addition to being incredibly low maintenance as far as animals, felines have demonstrated the ability to somewhat increase the rate of recovery in humans through the frequency of their purrs. They have also shown the marked ability of increasing the flow of endorphin's in humans with their appearance alone. It also allows me to monitor your vitals and responses in close range. It is, of course run through it's own artificial intelligence." the AI explained.
Natasha picked up the cat slowly, lifting it up and down. "It looks and weighs the same as a normal cat. Are you telling me this is a…catdroid?"
"Precisely, Agent Romanov."
Steve snorted and Bruce laughed softly. "Well, I for one am happy that this isn't a real cat. That's a little…less worrying, damage-control wise." Bruce said, stroking the cat that was now bundled in Natasha's arms.
"Um…am I the only one that's…you know, freaking out about this?" Clint asked. When Natasha raised an eyebrow at him, he sighed and ran a hand through his hair. "He built a cat. A cat so realistic that none of us even noticed. With an AI on top of that."
"So?"
"So…singularity?"
Tony started and both Bruce and Natasha recoiled. "Oh…" they murmured in unison.
At Steve and Thor's confused looks, "Skynet." Natasha supplied. Steve cringed.
"Uh…okay, well then. Just make sure you make things nice and peaceful when you take over the world, kay J? No killing people for shits and giggles or using them for fuel. I don't need some losers time hopping after my head, alright?"
"Yes, sir. Of course."
Steve sighed and ran a hand over his face. "You're supposed to tell him to NOT try to take over the world in the first place, Tony."
"Have you seen the idiots we've got in Congress? They'll thank him for it later."
Steve glared at him pointedly. "We would have to stop him, Tony."
Tony scratched the center of his hair with his forefinger. "It's fine if he waits until after we're dead, right?"
Steve's eyes narrowed at him, not even needing to voice the "stop encouraging him!" that expression held so well.
"Please rest assured, Captain Rogers, that should I come to the decision that it is necessary to take the managing of the world out of human hands I will be sure to keep many of them in the government and allow them to maintain the facade of control over the masses. …And ice cream will be free on Thursdays through government subsidies." the AI offered.
Steve buried his face in his hands and Clint grinned widely. "I am good with this plan. Are we sure we can't get the ball rolling on that now? To uh…make sure he does it the right way?" Clint asked, a little to excited for their captain's comfort.
Steve just sighed.