This is a behavior, similar to a grimace, which is known as the Flehmen Reaction. This is when your cat smells something, opens his mouth slightly, then wrinkles his nose and curls back his upper lip. In so doing, he is drawing in air, capturing the scent, and transferring it to a small, specialized sac known as the Jacobson's organ, an organ located in the roof of the mouth that sends signals about the scent to the brain. The Flehmen Reaction most often occurs in response to the scent of urine, though it can occur with other scents too.