We sweat for a reason. And yet, we spend $18 billion a year trying to stop or at least mask the smell of our sweat.

Yep, that's $18 billion a year spent on deodorant and antiperspirants. But even though you use it every day, we doubt you know all of these surprising facts about your swipe sticks.

Being anti-body odor is NOT a modern phenomenon.
ancient egypt
According to the New York Times, ancient Egyptians "invented the art of scented bathing" and took to applying perfume to their pits.

The first trademarked deodorant -- in 1888! -- was called Mum, and the first antiperspirant, Everdry, followed 15 years later, the Times reported.

Deodorant kills bacteria.
Sweat isn't inherently stinky. In fact, it's nearly odorless. The stench comes from bacteria that break down one of two types of sweat on your skin. Deodorant contains some antibacterial power to stop the stink before it starts, while antiperspirants deal with sweat directly.

Antiperspirants don't actually stop the sweating process.
The aluminum compounds in antiperspirants effectively stop up the eccrine sweat glands.

But the FDA only requires that a brand cut back on sweat by 20 percent to boast "all day protection" on its label, the Wall Street Journal reported. An antiperspirant claiming "extra strength" only has to cut down on wetness by 30 percent.

You really can become "immune" to your antiperspirant.
It seems that our bodies do adapt to the sweat-thwarting ways of antiperspirants, but no one really knows why, HuffPost Style reported. The body may adapt and find a way to unplug the glands, or simply produce more sweat in the body's other glands.

"It's a good idea to switch up your deodorant brand every six months to prevent resistance," Dr. Han Lee, assistant professor of dermatology at the University of Southern California, told Men's Health.

Your deodorant doesn't care if you're male or female.
Fun fact: While women have more sweat glands than men, men's sweat glands produce more sweat.

But deodorant for men or for women is most likely little more than a marketing ploy. In at least one brand, the same active ingredient is present in the same amounts in the sticks for men and women, Discovery Health reported. It's only packaging and fragrance that differs.

We're still falling for it, though: As of 2006, unisex deodorants make up just 10 percent of the sweat-fighting market, USA Today reported.

Not everyone needs deodorant -- and it's possible to tell if you do by your earwax.
Deodorant advertisers have done a pretty good job of convincing us that we're disgustingly smelly animals who need to be refined by their products.

But, most people don't smell as bad as they think they do, Esquire reported, and some, who come from a particularly lucky gene pool, don't even smell at all.

Short of forgoing all deodorant long enough to discover your true scent -- which this brave soul did for 10 days -- you can get an idea about your own personal smell factor by examining your earwax. (Hey, no one said this wouldn't be gross!) White, flaky ear gunk most likely means you could toss the deodorant stick. Dark and sticky wax... not so fast! Dry earwax producers are missing a chemical in their pits that the odor-causing bacteria feed on, according to LiveScience.

No one -- not even deodorant makers -- truly understands where those yellow stains come from.
The dominant theory is that the aluminum-based ingredients in antiperspirants somehow react with sweat or skin or shirts or laundry detergent or all of the above to make that foul stain. Hanes is even "researching the 'yellowing phenomenon,'" according to the Wall Street Journal. The only way to truly prevent them is to say no to aluminum-based antiperspirants.

You can make your own.
A number of plant oils and extracts contain their very own antibacterial powers, so in theory you can make your own stench-fighting deodorant relatively easily. However, people seem to find all-natural, store-bought products to have varying degrees of efficacy -- not to mention you won't find an all-natural antiperspirant, just odor blockers.

Also on HuffPost:

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  • Salt

    Ever wake up feeling a little puffy around the eyes? Too much salt can cause some of us to retain water, which can lead to swelling, say New York City dermatologist Dr. Neal B. Schultz. Because the skin around the eyes is so thin, he explains, the area swells easily -- and leaves you cursing last night's popcorn when you catch your reflection the next morning. "These effects of salt are definitely age related," he says, and become more common in middle age.

  • Shellfish

    Shrimp, crab, lobster -- and also certain leafy greens like seaweed and spinach -- are naturally <a href="http://ods.od.nih.gov/factsheets/Iodine-HealthProfessional/">high in iodine</a>, and a diet with too much of this element can lead to acne, says Schultz. However, "these breakouts are based on an accumulated amount of iodine over time, so there's no relationship between eating high iodine foods one day and breaking out the next," he says. Instead, he advises that people who are particularly acne-prone consume these foods a couple of times a month rather than a couple of times a week.

  • Milk

    Although its effects are probably still pretty small, according to Dr. Bobby Buka, a dermatologist also in practice in New York City, some <a href="http://www.mayoclinic.com/health/acne/DS00169/DSECTION=causes">dairy products may contribute to skin problems</a>. A 2005 study linked <a href="http://www.aad.org/dermatology-world/monthly-archives/2012/acne/diet-and-acne">higher milk consumption to presence of acne</a>. While the study had certain flaws, including the fact that participants were asked simply to <em>recall</em> how much milk they drank rather than record it in real time, more recent research, including a 2012 study in Italy, found a connection specifically between <a href="http://www.aad.org/dermatology-world/monthly-archives/2012/acne/diet-and-acne">skim milk and acne</a>. This is likely because of "a higher amount of bioavailable hormones in skim milk, since they cannot be absorbed in surrounding fat," explains Buka, which can then overstimulate the group of glands that produce our skin's natural oily secretions, according to the American Academy of Dermatology. In some people with rosacea, dairy products can also trigger the condition's tell-tale redness, Schultz says.

  • High Glycemic Foods

    Starchy picks like white breads, pastas and cakes, and even corn syrup, Buka says, are best avoided for dewy skin (and maybe even for <a href="http://www.huffingtonpost.com/2012/06/27/low-glycemic-foods-diet_n_1630893.html">maintaining weight loss</a>). Foods that are considered high glycemic can cause rapid spikes in blood sugar. A small Australian study from 2007 found that eating a <a href="http://ajcn.nutrition.org/content/86/1/107.full">low-glycemic diet reduced acne</a> in young men. But Schultz says there will need to be more research before we truly understand the relationship. However, if <a href="http://www.youbeauty.com/skin/food-acne">glycemic index</a> does prove to be related to skin problems, and you find yourself breaking out after eating something like French fries, it may be due to the starchy insides rather than that greasy, golden exterior, according to YouBeauty.com.

  • Sugar

    If starchy foods that break down quickly into sugar are an issue, it's no surprise that straight sugar can be problematic for the skin in much the same way. High blood sugar can <a href="http://www.dailyglow.com/photo-gallery/the-10-worst-skin-habits#/slide-7">weaken the skin by affecting tissues like collagen</a>, according to Daily Glow, and leave you more vulnerable to lines and wrinkles. Which is why it's likely not anything particular to <em>chocolate</em>, a <a href="http://www.huffingtonpost.com/2012/06/05/does-chocolate-cause-acne_n_1566076.html">rumored breakout culprit</a>, that's giving you trouble, but the high sugar content of that sweet treat. If you're worried about breakouts, but dying for a nibble, stick with the dark stuff -- it packs the most <a href="http://www.huffingtonpost.com/2012/03/28/chocolate-health-benefits_n_1383372.html">health benefits</a>, anyway.

  • Alcohol

    Alcohol is a <a href="http://www.huffingtonpost.com/2012/12/28/alcohol-effects-body-infographic_n_2333328.html">natural diuretic</a>, which means the more you drink, the more dehydrated you become. It saps the natural moisture from your skin as well, which can make those <a href="http://www.womansday.com/style-beauty/beauty-tips-products/foods-good-for-skin#slide-3">wrinkles and fine lines seem like bigger deals</a>, according to Woman's Day. It can also trigger rosacea outbreaks, Schultz says.