COPIED FROM WIKIPEDIA
"The flehmen response (English pronunciation: /ˈfleɪmən/, German: [ˈfleːmən]), also called the flehmen position, flehmen reaction, flehming, or flehmening (from German flehmen, meaning to curl the upper lip), is a particular type of curling of the upper lip in ungulates, felids, and many other mammals, which facilitates the transfer of pheromones and other scents into the vomeronasal organ, also called the Jacobson's organ.
In the flehmen response, animals draw back their lips in a manner that makes them appear to be "grimacing" or "smirking". The action, which is adopted when examining scents left by other animals either of the same species or of prey, helps expose the vomeronasal organ and draws scent molecules back toward it. This behavior allows animals to detect scents, for example from urine, of other members of their species or clues to the presence of prey. Flehming allows the animals to determine several factors, including the presence or absence of estrus, the physiological state of the animal, and how long ago the animal passed by. This particular response is recognizable, for example, in stallions when smelling the urine of a mare in heat. To detect estrus the male giraffe's flehman response includes the actual taste-testing of the female's urine."