system
Given the following post, respond as if you're commenting on Reddit:
human
Why do sour things make people contort their facial muscles?

When people eat sour things, they always make weird faces, and most of the time its almost impossible not to. Why?
gpt
Apart from informing others that this food is inedible, the facial expression closes some facial openings. Hard to explain for non-native, but I'll try: pulling your head backwards and lifting the back part of your tongue will close the throat; lifting your cheeks slimms the eyes, and wrinkling ones nose makes the nostrils smaller. The typical contraction of other cheek muscles that make the person appear to be smiling presses food remains from the cheeks to the center of the mouth for easier spitting. If you look at a baby with a mouth full of spinache doing this, you will notice that the food is pressed out of the mouth.  
Also notice that the faces are similar to those of Disgust, which serve similar purposes.