system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: Why do we cover our mouths when we are shocked by something?

When something very surprising happens, people cover their mouths with their hands.

Why do people do this?
gpt
Covering your mouth is a passive, not instinctive, action. 

Instinct is: shock > make noise to call for help or scare.

Secondary/ passive actions: realising you don't actually need help/ to scare means you just open your mouth. It's considered poor etiquette to have an open mouth in most cultures so you put your hand up to cover.

Having an open mouth can be seen as threatening (think a dog showing it's teeth or a baboon shrieking), hence the covering. Maybe the reason for us covering our mouths when we are shocked, or even when we yawn.

Very counter intuitive.