Be honest: Have you ever wondered what it really feels like to get kicked in the balls? Some dudes compare it to the pain of giving birth (um, no), while others say it's more like getting punched in the stomach by a professional boxer.

We'll never know for sure, but that doesn't mean we're not curious about other testicle mysteries. For example, why does one hang lower than the other? And what are "blue balls," anyway? To get answers, we spoke to a doctor who specializes in male sexuality. These nine family jewels facts will blow your mind—or make you glad you don't have a pair dangling between your legs.

Testicles are sperm factories

The testicles, also called the testes, produce about 1,500 sperm every second, Michael Reitano, MD, physician-in-residence at men’s health company Roman, tells Health. That's about 90,000 sperm every minute, 5.4 million every hour, and 130 million every day. Each guy is different, and sperm count does vary, but the typical pair of nuts make well over a million sperm every. single. day.

It only takes one sperm to fertilize a woman's egg, but the average ejaculation (which clocks in at about one-half to one teaspoon of semen) contains anywhere from 40 to 130 million sperm, Dr. Reitano says. If sperm isn't released through ejaculation, it's simply reabsorbed back into the body.

They're 5 degrees colder than the rest of the body

"If you think about it, the female has gonads as well, the ovaries, but why are they so well protected inside of the body when the male gonads are outside of the body and so vulnerable?" Dr. Reitano asks. The answer? Testicular temperature has to be about 5 degrees lower than the rest of the body in order to properly preserve sperm. The scrotum (the wrinkly pouch of skin that holds the testicles) is about 93.4 degrees Fahrenheit, while the body is typically 98.6.

The muscles in the scrotum actually act as temperature control for the testicles by moving them closer to or farther away from the body as the air gets warmer or cooler. For example, when the testicles get a bit too chilly, the scrotum will contract and pull them closer to the body for warmth. And if they can't take the heat, the scrotum will relax and let the testes hang low.

One hangs lower for a reason

The testicle on the left usually hangs a little lower than the one on the right, and it's no accident. "This allows the temperature of one testis to change without that energy being sent to the other testis, as it would if they were adjacent or touching," Dr. Reitano says. This way, the body can increase or decrease the temperature of one testis without affecting the temperature of the other. Now that's the kind of advanced heating/cooling system needed in just about every office building ever.

It's also normal for one to be a little bigger than the other. You know, just like boobs. Dr. Reitano says the right tends to be larger than the left.

They hurt like hell when they get hit

Okay, let's look at the facts: The testicles are crucial to reproduction, but unlike the ovaries, they're super vulnerable because they hang outside of the body. So how does nature protect them? Dr. Reitano explains that the testicles evolved to have a ton of nerves that are crazy sensitive to pain because this makes guys way more likely to protect them in case of danger.

The muscles in the scrotum will also pull the testes closer to the body if they sense any kind of danger...even if it's not always actual danger. Dr. Reitano says if you stroke your guy's inner thigh, you'll probably notice his scrotum brings the testicles nearer to his body. That reflex can also respond to emotions. "If a man feels threatened or fear, the muscles will [contract] without him even being aware of it," he adds..

The scrotum is as sexually sensitive as your vulva

We know the testicles are extremely pain-receptive, but they themselves aren't as sensitive to sexual pleasure as you might think. It's actually the scrotal sac, which holds the testicles, that's an erogenous zone. "The scrotum corresponds to the vulva and labia in females," says Dr. Reitano, putting it in perspective. There's no way to be 100% sure how similar the sensation is, but "we can estimate that the sensations are fairly similar in that we know the nerve distribution is almost identical," he adds. Playing with his scrotum alone probably won't make him orgasm...but it still feels pretty damn good.

Tight-fitting underwear takes a toll

A study published last year in Human Reproduction found that men who primarily wore boxer shorts averaged 25% higher sperm concentration and 17% higher total sperm count than men who wore other types of underwear. Dr. Reitano says he recommends loose fitting underwear (e.g. boxers) because it gives the testicles enough space to regulate temperature by moving closer to or farther away from the body. Sitting too long in a hot tub can also cause similar sperm count problems, since excess heat degrades sperm quality.

Testicular cancer is often caught late

Signs of testicular cancer include achiness or pain in a guy's back, groin, belly, or scrotum; abdominal bloating; a heavy feeling in his scrotum; a change in size of one or both testicles; or a lump on the testicles. Unfortunately, testicular tumors are often caught late because they can grow without compressing any other organs or bones nearby, making them relatively painless, Dr. Reitano says. The skin of the scrotum is so loose that a testicular tumor can grow without constraint.

Luckily, testicular cancer is highly curable, but it's still very important for guys to do self-examinations. (It's most common in men between ages 15 and 35, just FYI.) You can help him by taking matters into your own hands, literally. “Gently roll the testicle between the thumb and one or two fingers, and feel for a symmetric, round structure,” Michael A. Palese MD, chair of urology at Mount Sinai Downtown-Union Square in New York City, previously told Health. You want everything to feel "homogenous," he said, with no lumps or bumps.

One sometimes hides in a guy's body

When a boy baby is developing in the womb, his testicles typically drop from the abdomen to the scrotum. But in some cases, a newborn comes into the world with one or both testicles still in his abdomen. Sometimes the testicles will drop on their own in the first few months of life, but if it hasn't happened by the four-month mark, surgery will probably be needed, according to Mayo Clinic.

Though surgery can easily resolve the problem, it carries a risk of complications later in life. Dr. Reitano says men born with undescended testicles are 40 times more likely to develop testicular cancer. Risk factors for having a baby with hidden jewels include premature birth, low birth weight, family history of undescended testicles, and alcohol or cigarette use by the mother during pregnancy, according to Mayo Clinic.

Balls don't actually turn blue—but they can double in size

Men use the term "blue balls" to describe the painful aching they feel in their testicles when they become very aroused but don't get any release. Sure it's unpleasant, but we're not talking severe pain here...it shouldn't be anything more than mildly uncomfortable (for all the drama kings out there).