system
Given the following post, respond as if you're commenting on Reddit:
human
Eli5: Why do we automatically make the ‘sour face’ when eating sour food?
gpt
That's a good question - "sour" is how we describe the taste of acid. Generally the acids we eat come from two sources - 1) fruits like lemons, oranges, mangos, etc., 2) acids produced in fermentation like in kombucha, vinegar, some beers, and fermented dairy products. 

The sour face is an involuntary reaction which means it's wired deep into our brains and not a learned habit. If you give a new born a lemon they'll make the face without anyone ever having seen it before. 

The cause is likely 2 reasons -

1) it can aid in the release of saliva in your mouth to help dilute the acid which might have been harmful if too concentrated. 

2) its a social signal to others that the ingested food is sour, this can trigger further involuntary saliva production in people around you, helping them with the acidity, it can also signal to others, without language (again going back to primal instincts here) that the food is *wrong* the acidity can come from spoilage and the food is not suitable for consumption. The first person might get sick and die, but the others who witnessed the reaction know it was the food and to not eat it.