You may have wondered what that lighter patch is that appears on your underwear, or the underwear of someone with whom you are close enough to be rifling through their pants.
You are not alone in wondering this, as a post from the Vagina Museum in the UK has shown. Given the popularity of the thread, it appears it's a gap in people's education that needs correcting.
"I walked around all of my school life terrified when I changed I would get made fun of for this exact thing," one Twitter user wrote. "Sleepovers, gym, all of it was so stressful."
"We really weren't taught anything about our bodies, it really is unfair. We were just left to figure [this] stuff out," another wrote. "And most of the time, we end up being eaten up inside because we fear that something is wrong with us, but we're too embarrassed to speak up about it. It sucks."
Many chimed in to say that they had felt embarrassed about the patches, or that it was a sign that something was wrong. But, as the Vagina Museum points out, it isn't.
The underwear – taken from real people – is on display at the museum.
"We get a lot of feedback from people who didn't know that it happens, or why it happens or fel relieved to learn it wasn't just them," the museum wrote.
"It's not a sign of dirtiness or bad hygiene to have paler patches in your pants, it's something which happens when the acidity of a perfectly healthy vagina spends time in contact with fabric, and it's particularly noticeable on darker fabrics."
Having a (moderately) acidic vagina is protective against unhealthy bacterial and yeast infections. Your pH level will differ over your lifetime, normally at around 3.8-4.5 pH in the years you are menstruating, and slightly above that before puberty and after menopause.
"This discharge increases during ovulation and pregnancy due to an increase in cervical mucus," consultant gynaecologist Dr Alex Eskander explained to Metro.co.uk in 2019. "When exposed to the air, the discharge can stain underwear a mild yellow colour due to oxidation."
Some replied to say that holes had appeared in their underwear.
While others had some dubious tales and questions about teeth being bleached using the vagina method, or else tales of crowns falling out because of the acid.
"Even if vaginas were the right pH to whiten teeth (which they aren't) to do it would require constant contact between teeth and vag for a prolonged period," the Vagina Museum replied. "Which won't be much fun for your vagina. Don't do this, kids."
These myths, misconceptions, and gaps in knowledge are common, according to the museum.
"In a world where vaginas aren't talked about much, it's no surprise nobody passes on this information in school or at home, because many educators, parents and even healthcare professionals don't know these facts either," Zoe Williams of the Vagina Museum told IFLScience.
"The gynaecological anatomy is overlooked regularly, in research as much as society. For example, the first detailed anatomical study of the internal clitoris – an organ which is 10cm long! – wasn't published until 1998. People at all levels don't talk about vaginas and vulvas, due to a climate of shame and stigma. This means people can't gain a full understanding of their own anatomy, and allows myths and misconceptions to flourish."
The museum has run a Muffbusters: Vagina Myths and How to Fight Them exhibition focusing on dispelling some of the most widespread myths.
As for the claim about crowns below: