Excerpted from "What Do Dogs Know?":

Dogs and humans obviously differ in many ways. One of the most important differences is in how the two species perceive the world. For instance, dogs have an incredible ability to read scents. If you could unfold the inner surface of a dog's nose (the part with the cells that allow the dog to smell), it would actually cover a surface area larger than the entire extent of the dog's skin.

Dogs read the state of the world through their noses, and they write their messages to other dogs in urine. A particular dog's urine contains a lot of information about that dog. It smells different depending upon the dog's age and health; whether it is male or female, or a female in heat; and even depending upon the dog's emotional state. For a dog, sniffing a fire hydrant or a tree along a route popular with other dogs is a means of keeping abreast of current events.That tree is really a large newspaper containing the latest news items in the dog world, and perhaps even installments of classic canine literature.