|When your cat smells something and then opens his mouth slightly, wrinkles his nose, and curls back his upper lip, your cat is exhibiting what is called the 'flehmen reaction' or 'flehming.' He is drawing in air, capturing the scent, and transferring it to a small specialized sac called the 'vomeronasal organ' or 'Jacobson's organ.' This organ is located high up in the roof of the mouth, and has a large blood supply. It traps the odor molecules and sends signals to the brain regarding the scent.
Flehming can occur with a number of scents, but most frequently occurs when the cat smells urine. Many male cats will exhibit this behavior when they smell the genitals or urine of a female cat in heat.