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Flehmen response:

The flehmen response, also called the flehmen position, flehmen reaction, flehming, or flehmening (from German flehmen (of animals) meaning to curl the upper lip), is a particular type of curling of the lips in ungulates, felids, and many other mammals, which facilitates the transfer of odorant chemicals into the vomeronasal organ. In the flehmen reaction, animals draw back their lips in a manner that makes them appear to be “grimacing”. The pose, which is adopted when examining scents left by other animals of the same species, helps expose the vomeronasal organ and draws scent molecules back toward it. This behavior allows animals to detect odorants, for example from urine, of other members of their species. 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 most recognizable in stallions when smelling the urine of a mare in heat.

Courtesy of srah, from her comments in the first installment of this feature.