Dogs have a remarkable ability to discern and remember scents, far superior to that of a human. A dog's sense of smell may be as much as 100 times better than ours, making it the most sensitive and vulnerable part of its body. The human nose has only three-square centimeters of olfactory membrane where some dog breeds, such as Bloodhounds, have as much as 150 square centimeters of scent-sensing material in their noses. Dogs also have forty times more scent processing cells in their brains than humans.
Another factor for the dog's superior smelling ability is its ability to collect air in a special chamber of their nose. The air stays in the chamber when the dog exhales, so scent molecules accumulate there until there is a sufficient concentration available for the dog to identify an odor.
Besides being able to discriminate between different smells, a dog has a phenomenal olfactory memory. A dog can remember smells long after being exposed to the original odor. Not only can your dog recognize that a certain neighborhood cat passed by the day before, he can even tell what that cat had for dinner!
Dogs have an additional organ in their nasal cavity known as Jacobson's organ. This organ's function can best be described as a combination of taste and smell. Dogs can literally taste the air. A dog that is experiencing this taste/smell experience usually holds his mouth in a semi-open position that resembles a grin. Scientists call this reaction a Flehman Reaction. A dog uses his Jacobson's organ most frequently to experience sexual markers (urine) left behind by other dogs or when coming across the scent of a female dog in heat.