The Flehmen response in cats is a behavioral reaction where a cat curls back its lips, inhales air, and holds this posture for several seconds. This action helps them transfer scents to the vomeronasal organ, located on the roof of their mouth, enabling them to analyze pheromones. This behavior is often triggered by encountering strong, unusual odors and can indicate curiosity or the detection of pheromones related to mating or territorial boundaries. Essentially, it's a way for cats to gather more information about their environment or other cats nearby.