Flehmen Response

FMouth slightly open, upper lip curled back, teeth exposed — looking like a bizarre grimace. This isn't anger or aggression — the cat is using the vomeronasal organ (on the roof of the mouth) to analyze chemical information in the air. Cats show this when encountering novel scents: another cat's urine, unfamiliar food, or the scent of other animals on you. The organ specifically processes sex hormones and social chemical signals, so this “grimace” is actually the cat carefully reading a chemical letter.