Ever noticed your cat take a whiff of something (like your foot) and then make a face like they were totally offended! They hold their mouths open in sort of a grimace for a few moments. This is actually just a process of taking in the scents around them called the flehmen response.
The flehmen response is a particular type of curling of the upper lip in felines and some other animals, which facilitates the transfer of pheromones and other scents into the vomeronasal organ, also called the Jacobson's organ. This organ is located on the roof of the mouth and when your cat draws back their lips (the open mouth appearance that gives them that grimacing or smirking look) the vomeronasal organ is exposed and scent molecules can be drawn back toward it. In this way information such as the urine of another species can be detected and clues about the animals present environment can be determined. Things like the absence or presence of estrus and how long ago a predator or prey may have passed through are things a cat may use this scenting organ for. This organ is highly developed in cats accounts for cats having a far superior sense of smell than us humans:)! Other animals that utilize the flehmen response are tapirs, horses, tigers, buffalo, giraffes, goats, sheep, llamas, and kobs.