Cause they only focus on themselves? If that gets attacked, it’s an attack on their only asset. When your focus lies on a spectrum of interests other than yourself, you keep an overview and your overall self worth. I dunno.

Hmmm… That gives me an idea.

I know someone who might be a narcissist. Maybe I could exploit this vulnerability to expose him. You know, just to test my suspicions. Could just say something that mildly challenges his ego and see how he responds. If he menages to keep his cool like a normal person, then my hypothesis was wrong and I don’t have to be so suspicious of him any more.

What are you hoping to achieve? If you don’t like this person and they make you feel tense, unworthy, and like you’re walking on eggshells, you don’t need to “prove” they’re a narcissist. Even if they’re just an asshole, the solution is the same. Stop talking to them and leave.