Not like nipples, you're thinking of inverted.
Introversion. Maybe you've heard of it? Perhaps you haven't. (That's okay, why would you want to know anything about like... 50% of people anyway?) I'm not going to explain it to you, because there are 700 articles about it on every website lately and that's what Google is for anyway.
Regardless, I've been introverted since I can remember. Here I will provide some examples from my childhood experiences to give you proof, because no one who reads this has any reason to believe me.
Hopefully, if you know me, this doesn't come as a surprise. I'm predominately quiet, I observe more than I participate, and I quite literally NEVER want to talk to you at the soda machine, in the elevator, or at any time in my existence without a specified purpose. Small talk matters not.
I think I've given introversion a fair run, and I think it's time to give it a review. Let's do this thing.