[Bucky laughing sets John to grinning, he can't help it. He does try to tamp down on his own urge to chuckle, just knowing how much that's going to hurt. He puts on his most mock-severe expression.]

Bucky Barnes, you will wash your hair and brush your teeth tonight. I'm not running around with a bloke who doesn't put any sort of care into his hair or teeth. Cavities are real, and the greaser look went out of fashion a few decades ago.