Whether you've simply had a blackhead or two (or more!) or you just love to watch them being extracted on YouTube, you've probably had a lot of questions about them. So when Mental Floss’s favorite dermatologist, Dr. Sandra Lee (a.k.a. Dr. Pimple Popper), stopped by our offices last month, we asked her to tell us everything we need to know about blackheads—and then we sprinkled in a few fun facts of our own.

1. THE MEDICAL TERM FOR A BLACKHEAD IS “OPEN COMEDO.”

The word comedo comes from the Latin word for gluttony, and, according to the Oxford English Dictionary, was “a name formerly given to worms which devour the body.” It's been used since 1866 to refer to what we colloquially call blackheads, which C. H. Fagge described that year in On diseases of the skin, including the exanthemata as “A small worm-like yellowish black-tipped pasty mass which can in some persons be made, by pressure, to exude from hair follicles.”

“A blackhead is an open comedo, as opposed to a whitehead, which is a closed comedo,” Lee says. And it's not dirt in there: It's skin debris and oil. “It’s essentially a clogged pore with skin debris settling down within it,” Lee says. “When this environment becomes very positive for bacteria to flourish—namely Propionibacterium acnes—it can help to promote acne.”

2. THEY’RE NOT BLACK BECAUSE THEY'RE DIRTY.

It's because the debris inside the pore has been exposed to air. “It’s oxidized,” Lee says, “and it turns the darker color.” Closed comedones, because they’re sealed on top, remain white.

3. SOME BLACKHEADS CAN BE CAUSED BY THE SUN.

“There’s really two main categories [of blackheads],” Lee says. “The blackheads that are caused by acne when you’re a teenager,” which usually appear on foreheads, noses, and chins. And then there are solar comedones, blackheads and whiteheads that are caused by sun exposure. “Those we see in older patients, and they’re caused by a lot of sun exposure through your life,” she says. These typically appear around hair and the eyes, and, says Lee, “when you have them in mass distribution, it’s called Favre-Racouchot.” A classic case is the Masked Man (above).

“When you have a cluster, those are usually pretty good ones because the skin is a little more lax, so the blackheads aren’t stuck in there,” Lee says. “They tend to come out a little bit easier, and they tend to be bigger because people don’t really know what they are. They let them go. They don’t really pay attention to them.”

4. THEY CAN GET REALLY BIG.

A dilated pore of Winer is, essentially, a really, really big blackhead. They're named after L.H. Winer, who published a paper on them in the Investigative Journal of Dermatology in 1954. (“It is usually filled by a keratotic plug which, when removed, allows the further expression of a relatively small amount of white, soggy keratin,” Winer wrote. “The patient usually gives a history that the condition has been present for several years and that he frequently expresses cheesy material from the lesion only to have it refill within a period of a month.”)

“They’re like the unicorn,” Lee says. “I usually find those in older people, in hard to reach places.” Dilated pores of Winer usually appear on the face or trunk, but scientists found one rare case in a woman's ear canal. The strangest place Lee has ever seen a pore of Winer never made it into a video. “It was in [a woman's] pubic area,” Lee says. “She had no idea what this was, and she was scared. It was huge. It was so dry. There’s no way I would have been able to squeeze that thing out, so I had to cut it out.”

5. THERE ARE SOME PLACES YOU CAN’T GET BLACKHEADS.

In order to get a blackhead, you first need a hair follicle. So, Lee says, you won’t see blackheads in “areas that don’t have hair, like your palms and the soles of your feet.”