In all the years I have taught human sexuality courses to college students, no subject has generated more questions—from both men and women—than the female orgasm. Their questions run the gamut, ranging from the purpose of the female orgasm to the sex position that’s most likely to make a woman to climax. Lately, though, I’ve noticed the questions centering around one thing: the fluid some women release upon orgasm.

Most commonly, I get asked whether this can really even happen, which is interesting when you consider that we’ve known about female ejaculation for thousands of years. I mean, even the ancient Greeks and Romans wrote about it. In a recent paper documenting the history of female ejaculation, the authors described it well when they said it’s a phenomenon that seems to be repeatedly “discovered, described, and forgotten.”

Although I can’t say exactly where the recent surge in curiosity about female ejaculation stems from, I do suspect it’s tied to the growing fetishization of “squirting” in the world of porn. In fact, this is quickly becoming one of the most popular porn genres out there. So what do we know about female ejaculation? As it turns out, quite a bit.

For starters, when people say “female ejaculation” they aren’t necessarily referring to the same thing—it can come (pun intended) in two different forms. First, there are some women who, upon orgasm, ejaculate a small amount of whitish, milky fluid from their urethra. Its appearance resembles male ejaculate and, chemically, the two are similar—although, of course, the female kind doesn’t contain any sperm.

Female ejaculation just might be evolutionarily advantageous.

The source of this fluid is thought to be the Skene’s glands. These are tiny structures beneath the bladder that surround the urethra and consist of prostate-like tissue. Although the significance of this fluid is unknown, some argue that functionally, it has no purpose and, like the male nipple, it’s just a vestige from us all initially developing from the same set of structures in the womb.

Others, however, have theorized that this type of female ejaculation may serve a distinct purpose: protection from urinary tract infections (UTIs). The thought here is that this milky ejaculate is antimicrobial, which means that it would effectively cleanse the urethra during orgasm. To be clear, this is just a hypothesis; we don’t yet have evidence of it. However, if true, it would mean that female ejaculation just might be evolutionarily advantageous.

There’s another fluid that people often refer to as “female ejaculate” that has an entirely different appearance and origin. In this case, we’re talking about women who release a large amount of watery fluid upon orgasm, aka the “squirting” or “gushing” phenomenon.

Research has found that the fluid released during squirting originates in the bladder. In a 2015 study, a small group of women who reported previous experience with squirting were recruited. The study began by having these women empty their bladders, followed by an abdominal ultrasound. Then, they began stimulating themselves sexually (if preferred, they received stimulation from a partner). Once the women felt “sufficiently aroused,” they received a second ultrasound. Stimulation then resumed until orgasm occurred, at which point any fluid released was collected and a final ultrasound was performed.

What the results revealed was that the bladder filled between the first two scans and emptied between the second and third, thereby providing pretty conclusive evidence of the source of the fluid. In addition, chemical analyses revealed that the fluid itself was primarily urine, with traces of prostate-specific antigen (PSA) present in most samples. This suggests that, during squirting, there may be a small contribution from the Skene’s glands in some women.

According to the researchers, squirting can therefore be thought of as “an involuntary emission of urine.” To be clear, though, this doesn’t mean that women who experience squirting are incontinent. In other words, it’s not a sign that a woman has general bladder control issues or health problems.

Whereas some have argued that the milky kind of female ejaculate has an adaptive purpose, no such purpose has been proposed for squirting. However, some might argue that squirting could potentially serve a similar UTI-prevention purpose, especially in light of a few studies suggesting that women who urinate after sex have a lower risk of developing UTIs. This is purely speculative, though, and I should caution that some scientists have questioned just how effective post-sex urination is at preventing UTIs.

With all of that said, it’s worth mentioning that there’s an ongoing debate about whether both the milky and watery fluids should be called ejaculate. Given how different they are and the fact that they come from different sources, some researchers have argued that, technically, only the milky kind should be called ejaculate in order to avoid confusion.

There’s still a lot we don’t know about female ejaculation and squirting, including how many women are capable of each. Unfortunately, the few studies that have addressed this subject have provided estimates that are all over the map. What they do tell us though is that some women only seem to experience one of them, while others experience both. In fact, some women can experience both at the same time.

While we still have much to learn, what is clear is that female ejaculation and squirting are very much real, but they seem to be two different things that come from different places.

Justin Lehmiller, PhD is a sex educator and researcher at Ball State University, a Faculty Affiliate of The Kinsey Institute, and author of the blog Sex and Psychology. Follow him on Twitter @JustinLehmiller.