Most of us are equipped with five senses. We can see, hear, touch, taste and smell. With the holiday season just behind us, it's easy to see how these senses help us experience the world around us. There are definitely a lot of sights, sounds, tastes and smells associated with Halloween, Thanksgiving, Hanukkah, Kwanzaa and Christmas.

Let's concentrate on just one of the senses - smell. Think about some of the smells of the holiday season. There's the food - fresh baked turkey, ham, latkes and cookies and don't forget the peppermint candy canes. Then there's other stuff like pumpkins for Halloween, burning candles for Hanukkah and Kwanzaa, and fresh-made wreaths and fresh-cut trees for Christmas. Sometimes if you smell some of this stuff at a different time of year, it brings up a memory of the holidays. It's pretty obvious smell is a powerful sense. But how does it work exactly? It turns out it's pretty straightforward…

Smell is a very direct sense. In order for you to smell something, molecules from that thing have to make it to your nose. Everything you smell, therefore, is giving off molecules -- whether it is fresh baked cookies, your mom's perfume or a big, juicy orange or grapefruit. Those molecules are generally light, volatile (easy to evaporate) chemicals that float through the air into your nose. There are things that have no smell - like a piece of steel. Steel is a non-volatile solid. It has no smell because nothing evaporates from it.

At the top of your nasal passages behind your nose, there is a patch of special neurons about the size of a postage stamp. These neurons are unique in that they are out in the open where they can come into contact with the air. They have hair-like projections called cilia that increase their surface area. An odor molecule binds to these cilia to trigger the neuron and cause you to perceive a smell.

When you smell many fruits or flowers, what you are smelling is esters evaporating from the fruit or flower. Esters are organic molecules. For example, the ester that gives a banana its smell is called isoamyl acetate. The primary smell of an orange comes from octyl acetate. Esters can now be made artificially, and that is where artificial flavors come from.

Smell, Snake-Style Photo courtesy Saint Louis Zoo Emerald tree boa Like humans, snakes breathe airborne smells into nasal openings; but snakes have a secondary system, too. When a snake flicks its tongue, it is gathering odor particles for transfer to two fluid-filled sacs at the roof of its mouth -- Jacobson's organs -- that lead to a second, small olfactory chamber.

Humans can identify more than 10,000 different smells. We can do this because we have special receptors in the lining of our noses. Scientists believe that there are hundreds of different receptors. Each of these receptors are encoded by a specific gene. And, it's these specific genes that enable us to perceive certain smells. So, if your DNA is missing a gene or if the gene is damaged, it can cause you to be unable to detect a certain smell. For example, some people have no sense of the smell of camphor. Camphor is that stuff that's in some topical antiseptic sprays and those ointments (think little blue jar) that help relieve chest cold symptoms.

OK - here's an even better example. Have you ever eaten asparagus? And, afterward, have you ever noticed that your urine smells a little bit like the asparagus you ate? If not, don't worry - not everyone can make out the smell (but the smell is there, anyway). However, many folks do notice the odor. It's true. For those folks that can make out this smell, that means their DNA is equipped with the specific gene that encodes a sort of "asparagus-urine" receptor in their noses.