Experts estimate that dogs can catch a whiff of something that's one million times less concentrated than what humans can detect. With so much sniff power, it's hardly surprising that they stick their heads out out car windows. They could care less about the scenery. What they're after are smells. If you're driving through town at 30 miles an hour and your dog has his nose out the window, he knows where the bakery is, where the butcher shop is, which street leads to the local McDonald's, and maybe even what the mayor had for breakfast.

Dogs assume a characteristic expression when they put their faces into the wind: Their upper lips curl, their noses wrinkle, their eyes partly close, and their ears fold back. It looks as though they're experiencing a moment of ecstasy (which they probably are) but mainly they're concentrating. It's as though they're closing down all the rest of their senses to focus on this one.

There's a world of fascinating scents outside the car. This dog loves to hang her head out the window and sample every one of them. All dogs, from huge Great Danes to tiny terriers, have extraordinarily acute senses of smell. Their scenting ability is enhanced when they are moving quickly, which is one reason that they take advantage of open car windows.

Smells are so important to dogs that they have two separate systems for detecting them. One is the nose system. It consists of a huge amount of tissue called olfactory epithelium, which is loaded with scent receptors. This area takes up about 1/2 square inch in humans, but up to 20 square inches in some dog breeds. As air moves over the tissue, odor molecules settle in millions of scent receptors. The more air flow there is, the more scents dogs detect. A Dog's sense of smell is enhanced when they're moving quickly. In the evolutionary scheme of things, this probably made them better hunters because they could load up on scents while chasing prey.

Dogs have a second smelling system that's headquartered in their mouths. Near the upper incisors is a tiny duct that leads to a specialized gland called Jacobson's organ. It's designed to capture and interpret the most primitive types of smells.

Dogs depend on it to identify other dogs, choose a mate, and smell prey. When dogs scrunch up their faces in the wind, it looks like they're catching flies, but what they're really doing is catching scents.