One of the main ways that dogs communicate is with their sense of smell. They sniff other dogs to learn about their age, sex, and status. They can even tell a lot about a person's mood by the way he smells.

There's a powerful instrument that can detect tobacco wrapped in 27 layers of polythene or locate termites that are silently demolishing the foundations of a house. This instrument isn't a technological marvel created by humans, and you don't need an advanced degree to use it. It's the canine nose.

Among humans, the most important senses are sight and hearing. Among dogs, the sense of smell is paramount. A dog's sense of smell is up to a million times more sensitive than a human's.

Dogs can detect scents we don't even know exist, and they can identify the faintest of smells, even when they're heavily masked by other scents -- such as the odor of trace amounts of heroin that have been hidden in pungent aniseed. A dog's sense of smell is often more powerful than the best scientific instruments, which is why dogs have been used to detect not only drugs but also gas leaks and explosives, and to find people lost in the wilderness or buried in avalanches.

Dogs can smell things humans can't because they have more nasal membrane than we do. We have about 65 square inches of nasal membrane, while dogs have about 900 square inches -- an area that's greater than that of a dog's whole body, says Bruce Fogle, D.V.M., Ph.D., a veterinarian and author of The Dog's Mind. The nasal membrane is packed with olfactory receptors, specialized cells responsible for detecting scents.

A German shepherd typically has about 220 million olfactory receptors, while a human has about 5 million, says Mark Plonsky, Ph.D., a psychologist and dog trainer at the University of Wisconsin in Madison. It's believed that the bigger the dog and the longer his muzzle, the keener his sense of smell. German shepherds, for example, aren't just better at sniffing out scents than humans are, they're better at it than some other breeds. A fox terrier, for instance, has 150 million olfactory receptors, and a dachshund has about 125 million.

Dogs have an additional advantage. Their noses are always wet, as anyone knows who has woken up to the sensation of a cold, wet nose. It's believed that this sheen of moisture acts almost like Velcro, trapping scent molecules as they waft by. Along with the sticky mucus in the nasal passages, this allows dogs to collect and store large numbers of passing molecules.

Scents don't just drift conveniently into their noses. A dog's nostrils act like little antennas. Dogs wiggle them to collect scents and figure out where they're coming from.

When your dog raises his head and sniffs, he's breaking his normal breathing pattern to gather some new information. The air currents are abuzz with news, and he can hardly wait to tap into them and find out what's going on. "He has to actively sniff to pull the scent into the olfactory sensors, and when he doesn't, he's effectively turning his nose -- or his sense of smell -- off," says D. Caroline Coile, Ph.D., a neuroscientist in Ochlocknee, Georgia.

What all this means is that dogs have the ability to take in and identify scents that humans don't even know exist.

The scent molecules gleaned with each sniff are ultimately distilled and transported to various parts of the brain, much of which is devoted to remembering and interpreting them. Dogs have the ability to tap into this scent storage bank throughout their lives.

"Odors have a powerful influence on both the behavior and the physiology of the dog," says Dr. Fogle. "Smell memories last for life and affect almost all canine behavior." Scents tell them where they are, who a dog or person is, and even what state of mind that other creature happens to be in.

A dog who raises his leg on a tree isn't being indelicate. He's essentially pinning a notice on the community bulletin board. The scents in urine are as unique as the fingerprints among humans. Dogs who sniff trees, electric poles, and fire hydrants are gleaning tremendous amounts of information by reading the scent messages that other dogs have left.

The urine of female dogs in heat, for example, contains different pheromones -- scent molecules -- than that of dogs who are out of season. The males, of course, are eager for this information.

It's not only urine that contains scent signals, says Ian Dunbar, Ph.D., a veterinarian and author of Dog Behavior. The anal glands, stools, and saliva also contain olfactory information that dogs are keen to get hold of.

Even though dogs introduce themselves by sniffing each other's faces, it's the back ends that get the most attention. A quick sniff reveals a lot: how old a dog is, which sex, neutered or intact, relative or stranger. Scents also reveal a dog's confidence and social status, and what his mood happens to be at the moment. Dogs synthesize all of this information and figure out very quickly what their relationship with another dog is likely to be.

Although dogs do the sniffing routine longer with dogs they don't know, even housemates will sniff each other frequently. Behaviorists aren't sure why dogs continue to sniff even when they know each other intimately. It may simply be the dog equivalent of saying, "How are you today?" and catching up on the gossip.

A dog's fascination with smells doesn't stop with sniffing. Even dogs that have lived indoors all their lives appear to have an instinct that tells them to get dirty and roll in smelly things at the first opportunity. "It's camouflage," says Torry Weiser, a dog trainer in San Francisco. "What they're doing is using the scent of another creature to disguise themselves from something they're preying on and to get closer to it." Dogs don't have to think about prey and predators very much anymore, but the urge lives on. "They seem to be having a great time doing it," Weiser adds. "It's not perfume to us, but it certainly is to them."

Your dog knows your scent and has it filed in his memory, along with the smells of all the other people he's been introduced to. Some people your dog will remember with affection, others with fear and loathing -- and his "scent memory" will be triggered every time he meets them.

The one smell dogs value most is the smell of their owners. "It's a familiar smell that conveys comfort and safety," says Weiser. That's why experts recommend leaving an article of worn clothing with your dog when you have to leave for any length of time. The piece of clothing has your smell on it, and it gives comfort to him.

And like it or not, your dog can tell a lot about your mood just by your smell. A person's body odor is believed to change depending on his or her mood, and dogs are thought to be able to pick up on this.

Research has also shown that "happy tears" contain different chemicals than "angry tears," and some experts believe dogs can tell the difference -- and know right away whether to nuzzle your hand or give you a wide berth until you've calmed down.

Perfume, deodorant, cigarette smoke, and other odors that linger on skin and clothing all combine to make up a person's individual smell. Changing that composite smell "picture" in some way -- using a new perfume or none at all, for instance -- can confuse a dog and dull his ability to recognize someone as quickly as he normally would.

Dogs don't care if you're sweaty or have something pungent on your hands, but there are odors that do turn them off. Among these are citrus smells, such as lemon, lime, and orange, and spicy smells like red pepper. They particularly dislike the smell of citronella, which is why it's often used in spray form to keep dogs away from certain areas.

There are odors that turn dogs on, too, and often they're ones that turn us off. The trash smells like a smorgasbord to our dogs, but if we smell it at all, it's the opposite of delectable.

Sometimes, because of illness or accident, a dog's sense of smell can become impaired. "Dogs with a loss of smell seem to do just fine," says Dr. Coile. They'll tend to rely more heavily on their other senses to give them the essential information they need, she explains. They may start to eat less, though. The smell of their food is important to them, even more than the taste. That's why veterinarians often advise warming food when dogs lose their appetites. Warming food boosts the aroma, which may get them eating again.