system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: What makes a fake smile different from real one?
gpt
Humans are extraordinarily good at picking up very minute changes in facial expressions. Most of it happens unconsciously, that is to say automatically. Same kind of thing happens when you display emotions on your face. Tiny tiny muscles arrange the skin bits in very particular order to display an emotion, say a happy smile face. Im sure both of these things can be practiced to a point, but mostly it is all autonomous.

Fake smile is someone putting a smile-mask on top of some other emotion and it looks different because your brain does the brain-work automatically and the smile seems off.