You thought your fingerprint was unique?
Did you know that your sense of smell is even more unique? No two humans are alike in their sense of smell. The sense of smell (olfaction), is our process of detecting chemicals floating in the air. Chemicals enter the nose and dissolve in mucous (a membrane) called the olfactory epithelium. The olfactory epithelium is located about 7cm up into the nose from the nostrils. Humans have about 40 million olfactory receptors. This may sound like a lot, but some animals have as many as 40 BILLION olfactory receptors!
Modern science actually does not know what causes the olfactory receptors to work. But it is known that the electrical activity produced in the hair cells transmit to the olfactory bulb. From here, the olfactory tract transmits signals to the brain; the brain, being part of the limbic system, is involved with emotional behavior and memory. This is why the sense of smell can make you feel a certain way AND evoke memories!
The condition of having no sense of smell is called anosmia. About two million people in the U.S. have this condition. Some dangerous gases are odorless (such as natural gas) which is why the gas company adds a "rotten egg smell" to help us detect leaks! Your olfactory senses are as unique as your fingerprint, and you can detect several thousands of different smells.