Smart people

What is about people that are just perceived as smart. Like there’s this certain aura where you look at someone and you can tell they’re smart or they seem smart. And people say glasses braces like the generic thing but I’ve noticed there isn’t a specific “uniform” smart people have, like there was this guy in my class and he dressed like a “roadman” but you can just tell he was smart, people say I look smart but tbf I used to have glasses and braces but I don’t have braces anymore so is it that makes people look smart even tho they don’t fit a fixed appearance category