"The Flehmen response is a behavior in which an animal curls back its upper lip exposing its front teeth, inhales with the nostrils usually closed, and then often holds this position for several seconds." "The behavior facilitates the transfer of pheromones and other scents into the vomeronasal organ (VNO, or Jacobson's organ) located above the roof of the mouth via a duct which exits just behind the front teeth of the animal." (from Wikipedia)
