What's something you assumed was common knowledge until you found out most people had no idea?

[Peter]

There's a specific, slightly disorienting feeling when you casually mention something you've always assumed everyone just knows, and the room goes quiet, or someone asks a genuinely confused follow up question that makes it clear this was not, in fact, common knowledge at all

Sometimes it's a practical life skill, sometimes it's a random piece of trivia you picked up so early you forgot it wasn't universal, sometimes it's something specific to a job or hobby you've been doing long enough that the knowledge just feels like baseline reality rather than specialized information

Curious what everyone's version of this is, the thing you mentioned once expecting a nod of agreement and instead got a room full of blank stares