We generally don't pay a whole lot of attention to it, but our sense of smell has some important jobs to do. It helps us to distinguish between foods that are safe to eat and foods that are on the verge of becoming bacteria cultures. The sense of smell warns us of other kinds of danger, too — such as fire or a gas leak. Smells also give food their flavor; you couldn't taste very much without them.

How do we smell things? How are we able to detect thousands of different odors, from dirty socks to roses? Here are the five steps of smelling — or "olfaction," as it's also known. The diagram below will help you follow the steps.

Step 2: Lock and key En route to your lungs, smell molcules get caught in a gooey membrane inside the nostrils. This membrane, known as the "epithelium," is full of the neurons that carry smell receptors. Receptors are your body's tools for picking up smells; the receptors latch onto the smell molecules as you inhale. You've got about five million receptors altogether — so there's quite a welcoming committee for all those smell molecules that whizz up your nose.

Smell receptors are picky: They'll only hook up with certain smell molecules, in the same way that a lock will only be opened by a certain key. So, for example, some smell receptors will respond to dirty-sock molecules, and some receptors will respond to coffee molecules.

You have about 1,000 different kinds of smell receptors — but you can detect about 10,000 odors. How can you pick up so many odors if you've only got 1,000 types of choosy receptors?

Scientists figure that groups of receptors act together in different configurations to pick up odors. A leading olfaction researcher, Linda Buck, compares it to the alphabet: The alphabet contains just 26 letters, yet we have thousands of words and numerous complex languages to convey meaning. Similarly, with 1,000 types of odor receptors working together in different combinations, your nose can make codes for about 10,000 odors.

So, what happens after the smell receptors pick up smell molecules?

Each activated smell receptor triggers the neurons in your nose to send a signal — or impulse — to neurons in the part of your brain known as the olfactory bulb. But the work isn't done yet. The activity in the olfactory bulb is processed in an even higher level in your brain, in the place known as the "olfactory cortex."

Step 4: Brainpower Signals are relayed from the olfactory bulb to the olfactory cortex, triggering patterns of activity in the cortex. Scientists figure that certain patterns of activity correspond to certain smells (just like certain groups of letters form words). So you'll see one pattern for coffee and another for dirty socks.

This suggests that specific parts of the olfactory cortex respond to specific smells — an idea that scientists came up with only recently. It's like flicking light switches. One group of switches will go on for coffee. And another group will go on for roses.

What's more, research suggests that the same smells probably activate the same switches for everyone. So, if you and your friend both smell dirty socks, the same patterns of activity are going on in your respective olfactory cortexes. Your brains register what odor it is in the very same way.

This is actually a radical new finding in the study of olfaction, and it was made by scientists studying mice. In the Nature journal last November, leading researchers described how all mice are wired to decode smells in the same way. "The fact that this [smell] information is highly organized in the olfactory cortex, and is the same in different individuals, implies something about the perception of odors among different individuals," says researcher Linda Buck. "It provides a potential explanation as to why the odor of, say, a skunk smells bad to all people." Buck and her colleagues are making an "olfactory map" for mice and some day they expect to do the same for people. Their work is advancing the field of olfactory study by leaps and bounds.

The smelling process is complex, but in real time, it happens instantaneously. When an odorant coffee molecule makes its way into your nose, it takes a split second for you to consciously realize what the smell is. Your body and brain have the power to receive and interpret complex data at phenomenal speed.

The answer is no, because earthworms don't need a nose like ours to smell things. Earthworms can "smell" things with their entire bodies because they have chemical receptors all over their skin. Other members of the animal kingdom have various ways of picking up molecules in the air and on the surfaces they come into contact with. Snails can smell things with their tentacles and other parts of their bodies, including the front of the foot (the entire underside of a snail's body is known as its foot). Snakes pick up odors with their tongues and octopusses have smell receptors behind their eyes. Nature has found lots of ways of allowing different creatures to detect odors, depending on what kind of environment they live in.

This process of detecting chemicals in the surroundings is known as "chemoreception." (Chemo stands for chemicals, and reception for picking things up. Similarly, picking up light is called photoreception. Photo means "light." The light-sensitive cells in the eye — the rods and cones — are called photoreceptors.) Smelling is just one kind of chemoreception. Another kind of chemoreception is taste.

Smells Good, Tastes Good
Scientists call taste "contact chemoreception" because it involves putting the thing you're tasting right on the tongue which bears the taste receptors. (Smell, on the other hand, is "distance chemoreception" because you don't need to put something right in your nose to smell it.)

Taste receptors are less sophisticated than smell receptors. While smell receptors can deal with about 10,000 different odors, taste receptors register just four tastes: sour, sweet, salty, and bitter. You can detect a rich variety of flavors in your food because of your sense of smell.

• Try eating a particular food with your nose held closed. Do you notice a difference in the flavor compared with just eating normally?
• Notice that when you have a cold and your nose is stuffed up, food loses its flavor. This is because your sense of smell can't help out as fully as it usually does.

As you can see, your sense of smell serves many important purposes. Embark on an olfactory adventure: try paying more attention to the "smellscape" and see if you can find out how much your nose knows.