Folds in the skin can be a perfect storm for bacteria and odor to develop. Moist, warm, and dark, bodily hotspots like armpits can get ripe after exertion or intermittent bathing. Bacteria breaks down sweat, forming thioalcohols, which emit cartoon stink lines from your underarms.

Fortunately, modern civilization has taken to deodorants, which limit perspiration in the armpits by plugging up glands and killing bacteria that cause a loss of friends. The first mass-market deodorant, Everdry, was introduced in 1903, and its effectiveness came with a price. As it was made of harsh aluminum chloride, it was so acidic that it actually ate through clothing.

By the 1940s, deodorant solutions were less abrasive. Modern deodorants like Old Spice, Dove, and many others are far more agreeable. Pop the cap off a stick of deodorant, however, and you’ll notice there’s a second cap underneath. This one is more form-fitting, but its purpose is still a bit murky. Why not just use one cap? What makes deodorant so special a toiletry item that it requires two caps?

Look closely and you can see both the outer (blue) and inner caps.DonNichols/iStock via Getty Images

To find out, we reached out to Henkel Corporation, the makers of Right Guard deodorant. (Though not for much longer—Henkel sold both Right Guard and Dry Idea deodorants to Thriving Brands, LLC in June 2021.) According to Randi Melton, vice president of marketing in beauty care for Henkel, the inner cap is both a manufacturing and a quality control feature.

“Antiperspirants are filled from the bottom-up [or] upside down as a liquid in the manufacturing process,” Melton tells Mental Floss. “The liquid settles in quickly [into] the cap to keep the desired ‘dome’ shape of the stick.”

The dome is conveniently armpit-shaped to apply more even odor-neutralizing coverage. “The dome shape is desired because it fits with the shape of an underarm,” Melton says. “This enables application of the product to be smooth and to help apply appropriate distribution of the product on the skin.”

But that still doesn’t quite explain the need for two caps. According to Melton, the dual seal also works to prevent product tampering. Think of it this way: If someone took a deodorant off the shelf, applied it, and tried to repackage it, they couldn’t. Once the stick has been extended, it would be difficult to replace both the inner and outer caps without it being obvious. “You cannot ‘re-cap,’” Melton says.

If you find the plastic double cap stinks from an environmental perspective, a number of companies like Bai-Li and Battle Green are offering sustainable packaging that is either recyclable or refillable. Bonus: No current deodorant will eat through your clothing.

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