Of all the body parts, none may elicit more questions—and myths—than the penis. One of the male sexual organ's main roles is to make procreation happen, but the penis also has cultural significance. Here are 12 facts to clear up the confusion.

1. THE PENIS HAS TWO PRIMARY FUNCTIONS.

The penis has two main biological roles, according to Michael Reitano, a physician-in-residence and an expert in sexual health and wellness for Roman Health. One is the elimination of waste in the form of urine; the second is the means for transferring semen, which carries sperm from the testes out of the body to somewhere else, such as the vagina for procreation. Another of its functions is, of course, sexual pleasure.

2. IT DEVELOPS FROM A CLITORIS-LIKE ORGAN.

All mammalian embryos start life as female in presentation, before the chromosome process is activated, with an external, undifferentiated clitoris-like structure. Eventually, embryos with XX chromosomes will develop a clitoris, labia, and vagina, while those with XY chromosomes will grow a penis and testes.

3. THE PENIS IS COMPOSED OF THREE TUBES.

The penis may look like one long tube, but there are actually three columns of tissue that run along the inside of the penis. Two are corpus cavernosum columns, which extend from the base to the end of the penis and fill with blood to allow for erection. The other is the corpus spongiosum; it surrounds the urethra, the tube through which urine and semen passes. The corpus spongiosum also fills with blood during erection, but remains pliable to keep the urethra open.

4. HUMANS MAY HAVE THE LARGEST PENIS OF ALL PRIMATES ...

When girth is considered, the human penis is quite a bit larger than those of its primate cousins. "A gorilla, for example, has a penis just 2 inches long. Human males walk upright, and it is thought that larger genitalia might have acted as a sexual attractant in competitive situations," Reitano says. But according to the Center for Academic Research and Training in Anthropogeny, chimpanzees and bonobos' penises are more slender but comparable to the length of an average human penis (which is 5.16 inches long and 4.59 inches around when erect, 3.61 inches and 3.66 inches around when not). Scientists theorize that our unique proportions are a result of natural selection through female mate choice.

5. ... AND THOSE PENISES MAY ONCE HAVE BEEN BARBED.

The human penis is most definitely among the smoothest in the animal kingdom. "In the distant past, it probably had sharp barbs, like some of our primate cousins, to make sex with another partner soon after coitus unlikely," Reitano says. Chimp penises still have small barbs that "hold the female in place, and when the penis is removed, it irritates the female's vagina so she avoids other chimps who might want to mate with her," he explains. (Let's not get started on the exploding corkscrew penis of the Muscovy duck, which looks like it makes for some irritating coitus.)

6. IN THE 15TH CENTURY, MEN FEARED WITCHES WHO COULD STEAL THEIR PENISES.

One of many bizarre beliefs put forth in the Malleus Maleficarum, a 15th-century German witch-hunting manual by Heinrich Kramer and Jacob Sprenger, was the opinion that witches could steal men's penises. Kramer wrote that witches "can take away the male organ." He didn't mean Lorena Bobbitt-style, clarifying, "not indeed by despoiling the human body of it, but by concealing it with some glamour."

In a 2002 paper published in the Journal of Folklore Research, Moira Smith pointed to male sexual insecurity as a driver of the witch hunts. "Many of the crimes (maleficia) attributed to witches concerned sexuality: copulation with incubus devils, procuring abortions, causing sterility and stillbirth, and impeding sexual relations between husbands and wives," she wrote.

7. ERECTIONS ARE MORE COMPLICATED THAN THEY SEEM.

Achieving an erection is one of the most complex functions to happen in a man, Reitano says: "For starters, hormones must be released on demand, arteries need to carry six times more blood to the penis with perfect efficiency, the nervous system must transmit its signals without a hitch, and the mind must be working in perfect harmony with the body."

The ability to get and sustain an erection, he says, depends upon "a body that is perfectly tuned physically, psychologically, and emotionally." The inability to achieve an erection, a.k.a. erectile dysfunction, is usually the first sign of poor health, according to Reitano.

8. UNLIKE OTHER MAMMALS, HUMANS LACK A PENIS BONE …

Many mammals, including gorillas and chimpanzees, have a penis bone or "baculum," says Arash Akhavein, a urologist at Cedars-Sinai in Los Angeles. In some animals, like whales, the baculum hangs back inside the abdomen until mating time, and then it slides into the penis to help maintain an erection. Instead of relying on this kind of structure, the human penis requires blood flow and engorgement for erection.

9. … AND YET IT'S POSSIBLE TO "FRACTURE" A PENIS.

Unfortunately, the penis can be fractured during sex, Reitano says. While penile fracture is relatively uncommon, it can happen when the thick sheath called the tunica albuginea, which gives an erect penis its rigidity, is injured by blunt force. "When it fractures, there is usually a clear popping sound, extreme pain, and the rapid loss of erection," Reitano says.

And yes, researchers have studied the sexual positions associated with the problem in heterosexual couples. A 2014 study in Advances in Urology found that the woman being atop the man was the primary position associated with penile fracture; the man being behind the woman was the second most commonly linked. A 2017 study in the International Journal of Impotence Research found that man-behind-woman was most often associated with fracturing a penis, followed by man-atop-woman.

10. MASTURBATION FEARS MAY HAVE DRIVEN MASS CIRCUMCISION.

In an uncircumcised penis, the glans (head) of the penis is covered by skin known as the foreskin, which can be pulled back from the glans. Circumcision, the removal the foreskin, is an ancient surgical practice performed for religious reasons and to prevent issues stemming from poor hygiene.

One thing circumcision doesn't prevent is masturbation. But in English-speaking countries in the 19th century, a general attitude against sexual profligacy, fueled by religious ideology, led to fears about the effects of masturbation and a spike in circumcision. Religious leaders and physicians warned people to avoid "self-abuse" for fear it could cause physical and mental disorders such as tuberculosis, memory loss, and epilepsy. As Tom Hickman writes in his book God's Doodle: The Life and Times of the Penis, "What made circumcision common among the proliferating 19th-century middle classes on both sides of the Atlantic was the hysteria about masturbation; removing the foreskin helped its prevention, doctors declared, and also cured bed-wetting and other conditions."

11. ADULT MEN GET CIRCUMCISED, TOO.

It's most common for boys to be circumcised as newborns, and in some cultures as adolescents, but there are a number of reasons why adult men may choose to have a later-in-life circumcision, Akhavein says. These include tears in the skin where the tip of the penis attaches to the foreskin; when the foreskin is too tight to be retracted easily, a condition called phimosis; and a buildup of smegma, a whitish waxy substance made up of dead skin cells and oils. Men who had an incomplete circumcision (too little skin removed) in childhood would also be good candidates.

12. ICELAND HAS A PENIS MUSEUM.

In 1974, an Icelandic history teacher named Sigurður Hjartarson received a cattle whip made out of a bull penis, which apparently gave him the idea to collect other penises, because hey, why not? The result is the Icelandic Phallological Museum, which holds a collection of more than 238 penises and penile parts from nearly all the fauna in Iceland. Yes, including Homo sapiens.

The largest penis on hand, from a sperm whale, stands at 6 feet tall and weighs nearly 150 pounds. "You’ll learn that as with everything in nature, the diversity in this department is as great as in any other; even within the same species the difference in size and shape is often quite remarkable," Hjartarson told Mental Floss in 2015.