The rat's sense of smell registers not only average scents, but also the presence of chemicals that denote a change in atmosphere or emotion. Rats breathe first through their noses. Air then passes past a patch of skin rich with smell receptors and olfactory neurons, which are tipped with hair-like cilia. Smell particles, also known as odorants, bind to the cilia, triggering a neural response.
Rats also have a second smell organ, called the vomeronasal organ, or VNO. The VNO is set in a thin, ovoid space in the lower portion of the nasal passage, beside the septum. When rats sniff and taste, scent molecules dissolve and make their way to the VNO receptor. The VNO is used primarily to detect pheromones transmitted between two or more members of the same rat species. These pheromones are present in scat, urine and glandular secretions of rats and are detected through sniffing or licking.