Cats rely heavily on their sense of smell. A cat will always sniff its food before eating and scent marks are an essential part of feline communication.  Cats also have a unique mechanism at the top of their mouth which enables them to make a special analysis of air molecules. A pair of organs called the Jacobson's organ or the vomeronasal organ lets the cat analyze air that is inhaled through the mouth rather than the nose.

What is the Jacobson’s organ?
Cats (along with snakes and a few other mammals) have a magnificent organ called the vomeronasal organ most commonly known as the Jacobson's Organ. It is located inside the mouth just behind the front teeth and connects to the nasal cavity. Opening his mouth slightly enables the Jacobson's Organ to open up the ducts connecting to the nasal cavity. The appearance of the cat as he brings air into the Jacobson's Organ has sometimes been likened to a smile other times to a grimace. The Jacobson's Organ seems to play a large part in the sense of smell of a cat.

When a cat uses this special mechanism she curls back his lips opens her mouth and seems to grimace with a smile or an expression of disgust. This is called the “flehmen” reaction and is seen in connection with special scents that the cat wants to check thoroughly.

There are three hypotheses on the exact function of the Jacobson's Organ. The first is to perceive the smell of food; the second as a "sixth sense" to help predict unusual occurrences (such as earthquake volcanic eruption etc) and the third is for perception of sexual odors – pheromones.