There is something about wearing a T-shirt that almost nobody else owns and only 1 in a hundred people understands. I have one with an image of a square box that is labelled Schrodinger's cat. I barely understand it. But it's a hell of a thrill when the barista laughs out loud and we share a moment of mutual genius.
How different is this? Not so long ago being cool was choosing what everyone else was choosing, wearing, drinking and smoking. Ultimately, cool was defined by Madison Avenue. You knew it was time to change brands when Pepsi became the new cool, and a generation shifted.
But as the mass media has withered the new cool is defined not by some ad campaign but by the personal and intimate. Cool is about something being obscure enough to be different but not so obscure as to be meaningless. It's a bit of a balancing act. If none of your friends get it on Facebook it's not cool. But if they all do it is even worse.
It is great to arrive at a party with a new Chilean beer, provided someone else at the party also knows how good it is. A T-shirt with an image of long dead Italian actor is cool, provided someone else knows who they are. We want enough people in our tribe not to be lonely, but not so many as to be overcrowded.
And when it works, it really works. It starts when you find something which truly speaks to you -- reflecting your values/humour/life-experience. You laugh at the inside joke and you buy it. You wear it and you establish a shared connection with someone you have never met because they also get the joke. Your day, and theirs, have become a little richer over something as silly as Schrodinger's Cat.