The Jacobson's Organ

Cats have a specialized scent organ located in the roof of the mouth jsut behind the front teeth. It is called the vomeronasal sac or more known as the Jacobson's organ. Jacobson's organ is a small pouch lined with receptor cells and connects to the nasal cavity. When a cat smells certain scents, especially muskly or sexual ones,  it stretches its neck, opens it mouth and curls its upper lips.

This reaction is called Flehmen Reaction. The tongue gathers minute particles of scent and then is pressed to the roof of the mouth, transferring the scent molecules to the Jacobson's organ. Opening the mouth slightly enables the ducts to open up, connecting to the nasal cavity. The appearance of the cat as it brings air into the Jaconson's organ looks like a smile or grimace. We often see this in Kusky and Kurimu, especially when they were loitering around the car porch, sniffed around and ended up opening the mouth half wide with the blurry kind of facial expression.

 That's how cat looks like when it smells something....