system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5:What causes the reaction our faces have when we eat sour things like lemons?
gpt
Because it's an automatic 'disgust' response. Expressions are very helpful when eating, as it enables others around you to see what you're eating, and then avoid it if you look disgusted/revolted/etc. 

Edit: It's an evolutionary trait - if you study other primates in the wild, they do the same thing.