The flehmen response is a behaviour whereby an animal curls back its upper lips exposing its front teeth, inhales with the nostrils usually closed and then often holds this position for several seconds.
It may be performed over a site or substance of particular interest to the animal (e.g. urine) or may be performed with the neck stretched and the head held high in the air. Flehmen is performed by a wide range of mammals including ungulates and felids.
The behaviour facilitates the transfer of pheromones and other scents into the vomeronasal organ located above the roof of the mouth via a duct which exits just behind the front teeth of the animal. Reference: Wikipedia