system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: Why does the human smile convey happiness? Shouldn't stretching the muscles in your face upwards to display your teeth be threatening like it is with animals?
gpt
No, because we don't bite things to hurt them. Animals like dogs have their teeth as their main weapons, so they show them when they're angry to remind other animals that they can fight back. The human equivalent would be flexing your biceps or showing a weapon, because we use blunt force and tools instead of biting.