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It's the first sense you use
when you're born.

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One out of every fifty of your genes
is dedicated to it.

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It must be important, right?

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Okay, take a deep breath
through your nose.

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It's your sense of smell,

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and it's breathtakingly powerful.

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As an adult, you can distinguish
about 10,000 different smells.

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Here's how your nose does it.

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Smell starts when you sniff molecules
from the air into your nostrils.

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95% of your nasal cavity

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is used just to filter that air
before it hits your lungs.

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But at the very back of your nose

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is a region called
the olfactory epithelium,

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a little patch of skin
that's key to everything you smell.

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The olfactory epithelium has
a layer of olfactory receptor cells,

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special neurons that sense smells,

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like the taste buds of your nose.

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When odor molecules hit
the back of your nose,

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they get stuck in a layer of mucus
covering the olfactory epithelium.

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As they dissolve, they bind
to the olfactory receptor cells,

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which fire and send signals
through the olfactory tract

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up to your brain.

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As a side note, you can tell a lot

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about how good
an animal's sense of smell is

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by the size of its olfactory epithelium.

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A dog's olfactory epithelium

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is 20 times bigger
than your puny human one.

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But there's still a lot we don't know
about this little patch of cells, too.

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For example, our olfactory
epithelium is pigmented,

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and scientists don't really know why.

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But how do you actually tell
the difference between smells?

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It turns out that your brain has

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40 million different
olfactory receptor neurons,

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so odor A might trigger

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neurons 3, 427, and 988,

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and odor B might trigger

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neurons 8, 76, and 2,496,678.

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All of these different combinations

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let you detect a staggeringly
broad array of smells.

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Olfactory neurons are always fresh
and ready for action.

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They're the only neuron in the body
that gets replaced regularly,

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every four to eight weeks.

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Once they are triggered,
the signal travels through a bundle

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called the olfactory tract

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to destinations all over your brain,
making stops in the amygdala,

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the thalamus, and the neocortex.

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This is different from how sight
and sound are processed.

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Each of those signals goes first
to a relay center

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in the middle of the cerebral hemisphere

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and then out to other
regions of the brain.

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But smell, because it evolved
before most of your other senses,

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takes a direct route
to these different regions of the brain,

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where it can trigger
your fight-or-flight response,

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help you recall memories,
or make your mouth water.

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But even though we've all got
the same physiological set-up,

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two nostrils and millions
of olfactory neurons,

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not everybody smells the same things.

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One of the most famous examples of this

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is the ability to smell
so-called "asparagus pee."

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For about a quarter of the population,

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urinating after eating asparagus

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means smelling a distinct odor.

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The other 75% of us don't notice.

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And this isn't the only case
of smells differing from nose to nose.

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For some people, the chemical
androstenone smells like vanilla;

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to others, it smells like sweaty urine,

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which is unfortunate

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because androstenone is commonly found
in tasty things like pork.

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So with the sweaty urine smellers in mind,

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pork producers will castrate male pigs

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to stop them from making androstenone.

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The inability to smell a scent
is called anosmia,

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and there are about 100 known examples.

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People with allicin anosmia
can't smell garlic.

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Those with eugenol anosmia
can't smell cloves.

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And some people can't smell anything

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at all.

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This kind of full anosmia
could have several causes.

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Some people are born
without a sense of smell.

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Others lose it after an accident
or during an illness.

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If the olfactory epithelium
gets swollen or infected,

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it can hamper your sense of smell,

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something you might have experienced
when you were sick.

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Not being able to smell anything
can mess with your other senses, too.

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Many people who can't smell at all

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also can't really taste
the same way the rest of us do.

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It turns out that how something tastes
is closely related to how it smells.

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As you chew your food,
air is pushed up your nasal passage,

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carrying with it the smell of your food.

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Those scents hit your olfactory epithelium

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and tell your brain a lot
about what you're eating.

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Without the ability to smell,
you lose the ability to taste

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anything more complicated
than the five tastes

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your taste buds can detect:

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sweet, salty,

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bitter, sour, and savory.

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So, the next time you smell exhaust fumes,

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salty sea air, or roast chicken,

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you'll know exactly how you've done it

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and, perhaps, be a little more
thankful that you can.