The aroma of fresh popcorn, the smell of coffee brewing or of wood smoke on a frosty evening--these scents enliven and enrich our world. Perhaps because it is our oldest sense and connects to our oldest brain structures, smell most strongly triggers memories and emotions.

Your sense of smell begins when you inhale, bringing chemical particles into your nasal cavity. Here, millions of smell sensors are packed into a patch smaller than a postage stamp. These chemicals have to be dissolved to be sensed, so your nose produces sticky mucus. When these dissolved chemicals come into contact with smell receptors, nerve impulses are sent to the brain.

Q. Why do smells fade quickly?

A. When you fist inhale the scents of the forest--pine needles, musty leaves, split wood--you smell everything at first. But after a while, strong scents begin to fade. That's because smell sensors get used to the scents and stop sending new signals to your brain. This helps you become aware of newer smells, like the odor of a skunk. The following activity demonstrates how smell sensors lose sensitivity:

MATERIALS

• Cotton ball
• Lemon juice
• Film canister with a lid
• Clock with second hand

Soak the cotton with lemon juice. Then squeeze it out, place it in the film canister and snap on the lid for a few seconds. Check the time and remove the lid; hold the canister under your nose and sniff. Breathe deeply until you no longer smell the lemon. How long were you able to detect it?

Q. Why don't I immediately smell room deodorizer when it's sprayed?

A. Scents are carried on the air, so it takes time for it to reach you if it's sprayed some distance away. City planners try to place factories where breezes carry their odors away from where people live.

Q. Why can't I smell as well when I have a cold?

A. Extra mucus is produced to help flush germs out of your nose when you have a cold. When too much is produced, the smell sensors are buried, preventing dissolved chemicals from reaching them.

FACTOIDS

• Houseflies smell with their feet and wings.
• Snakes smell by flicking their tongues and pressing captured molecules against receptors on the roofs of their mouths.