Folds in the skin can be a perfect storm for bacteria and odor to develop. Moist, warm, and dark, bodily hot spots 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 antiperspirants, which prevent sweating in the first place by blocking sweat glands, and deodorants, which kill the bacteria and mask any remaining odors that might cause a loss of friends. 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? It turns out that antiperspirants are filled from the bottom up, or upside down, as a liquid in the manufacturing process. The liquid settles quickly into the cap to keep the desired “dome” shape of the stick. The dual seal 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. So, there you have it. Now you have a conversation starter for the next time you're feeling awkward at a party.
