Whenever you're home alone:
Steve:
Whenever you were home alone in yours and Steve's shared apartment you would have little dance parties. Just goofing off, relieving stress, having a good time. Steve would come home to your dancing and immediately join in. He treasured those moments.
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Bucky:
Whenever you were home alone you would get excessively bored. You would clean, watch Netflix, bake a (horrible tasting) cake, even create a parkour course in the living room. Bucky would always come home to something strange, whether it was you sweeping the ceiling or you wrapped in blankets on the floor. Bucky would always scramble to get a camera to capture the moment.
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Tony:
Whenever you were home alone you would mess with Tony's suits. You would put them on, pretend to shoot them, even pretend to be him. This was all recorded and later used as blackmail.
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Bruce:
You would pretend to be the hulk and fuck around, throwing pillows and stuffed toys around. Bruce never caught you or at least said he caught you.
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Thor:
You would play with the hammer, you could use it but you never told Thor, you didn't want to crush his ego. You would throw it around and see what you could do with it. Thor only started asking questions when he noticed the Hammer was always in a different place when he came home.
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Loki:
You would wear his helmet, pretending to be him in the mirror. He almost always caught you. He would either pout for the rest of the day or mock you, depending on what he caught you saying.
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Clint:
You would play with the weapons he left, mainly the blunt ones. You would pretend to be a ninja, beating up stuffed toys. He eventually caught you, promising to teach you how to properly wield the weapons.
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Natasha:
You would play with her knives. She had so many and they were so pretty, you couldn't help yourself. You would throw them at toys or cutting boards, test their sharpness, anything to ease the boredom.
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Wanda:
You would go through her closet, seeing what clothes she already had and taking back yours. You would use your knowledge to get her birthday and anniversary gifts. She eventually caught on.
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Pietro:
You would tie all of his shoelaces together, every single pair. It was hilarious watching him face plant after putting them on. He never realised it was you doing it, always blaming it on one of the other guys.
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Peter:
You would go through the formulas he had for the web fluids and fix them, making them stronger- safer. He realised it was you after two years.
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T'challa:
You would put on his suit, quoting him in the mirror. He always caught you but never told you. He found your antics unbelievably adorable.