Your health teacher might have spent forever talking about the symptoms of sexually transmitted infections, but glossed over the stuff you really wanted to learn about — like, how penises work, what erections feel like, and if a guy's penis can ever — eeeeeeek! — break.

Ellen Kate Friedrichs, a health and sexuality educator in Brooklyn, NY, breaks down what you always wanted to know but were too embarrassed to ask.

1. What are blue balls?

When a guy gets turned on but doesn't come, he can sometimes feel an ache in his testicles that's referred to as blue balls. His genitals don't actually turn bright blue, but there can be a slightly blue tinge.

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You should never feel pressured into hooking up with a guy or going further than you're comfortable with just because he complains of blue balls. He can easily deal with the problem on his own, and the discomfort isn't so terrible — on a scale of one to 10, it feels like a three or a four.

While he might complain about it, Friedrichs says, "This is not dangerous and he sure doesn't need to have sex with a partner to deal with it. Either he can do something non-sexual until it goes away, or he can masturbate."

And don't ever let a guy tell you blue balls are your fault. Bodily functions are no one's fault. They just happen!

2. What does an erection feel like?

If you don't have the equipment, it's only natural to be curious. "A lot of guys describe erections as feeling full and sensitive in a really pleasurable way if they happen at the right time," Friedrichs says. "But an erection can start to feel pretty uncomfortable if it turns up at an unexpected time. Say, like in the middle of math class!" #Awkward.

3. Why are some guys circumcised? What's the difference between a circumcised and uncircumcised penis?

"Being circumcised just means the foreskin covering the head of the penis is removed," Friedrichs explains. "The number of baby boys who are circumcised in the U.S. ranges from about 40 to 60 percent depending on the decade."

Some parents choose to circumcise their sons for religious reasons (it's traditional for Jewish and Muslim families) or cultural reasons (it's a widespread practice in the U.S.).

Imagine the difference between a neck covered by a turtleneck versus a bare neck, and you have a pretty good picture of what the difference is between the two. When it comes to hooking up, guys who have their foreskin intact are slicker and more naturally lubricated than guys who are circumcised. And while some people fear circumcision reduces sensitivity, one recent — but small — study found that's not true.

"The only thing is, if a guy isn't cut, he should just make sure to pull back his foreskin when he takes a bath or shower and wash around the head of the penis," Friedrichs says.

4. Why does a guy need an erection in order to have vaginal sex?

"Penetration is generally easier if a guy has a full erection," Friedrichs says. "Having an erection is super important for using a condom properly, so that it doesn't fall off."

5. Do condoms really reduce guys' sensation during sex?

Some guys (garbage guys, we might add) will try to get away with not using condoms because they claim sex doesn't feel as good when they wear them. "But enough guys have great sex using condoms that I would argue that even if they affect sensation a bit, a lot of guys don't feel like condoms get in the way of great sex," Friedrichs says.

Most importantly, condoms are the only way to protect yourself from sexually transmitted infections, plus they're 82 percent effective at preventing pregnancy when used properly.

If a guy prioritizes sensation during sex over health and safety for both of you, that's a huge red flag. Stay away from that guy!

6. Can a penis ever break?

Weirdly enough, boners don't contain any actual bones. So, no, you can't break a penis.

"But don't ever bend an erection," Friedrichs warns. "That can cause real damage to the tissue of the penis."

The resulting injury can include painful bruising and swelling, which sounds like one of the worst things that could happen to a guy. Yikes.

7. If you're hooking up with a guy and he doesn't get hard, does that mean he doesn't like you?

When guys are super turned on, they often get hard, but that doesn't necessarily mean he's not into you if he doesn't. "It's also just as likely that he is nervous or tired or distracted," Friedrichs says.

Remember, guys can feel self-conscious or nervous about their bodies, too, especially when they're hooking up. If he's not hard, that's not a surefire sign he's not into you. After all, he's making out with you — that counts for something! And if you're feeling insecure or preoccupied with something else, you might not be feeling it, even if you really like the person, and guys are no different.

8. What are wet dreams and why do guys have them? Do girls have the equivalent?

If a guy ejaculates in his sleep, it's called a wet dream. They're sometimes a result of a sexy dream, but not always, and more common during puberty.

"Girls can also orgasm in their sleep," Friedrichs says. "But we don't hear about this as much simply because unlike the evidence of a wet dream left by semen, if a girl had an orgasm in her sleep, it usually isn't as obvious in the morning."

9. What size is the average guy?

There's a range of sizes, especially for teens who are still developing. Plus, as Friedrichs adds, "The size of a guy when he is soft isn't an indication of how big he will be when he's hard."

By the time men reach adulthood, the average size is approximately 3.6 inches long when soft and 5.2 inches long when hard, according to one study of 15,000 men.

10. Why are guys so obsessed with penis size?

You've heard their jokes. What's the big deal? Friedrichs points to porn. "You rarely see a guy with a small or even average-sized penis in porn, so huge penises seem normal and almost expected," she says. "Plus, there's a myth that a big penis is needed to satisfy a partner. It isn't."

For the majority of girls, vaginal penetration might feel good, but doesn't help them have orgasms. Clitoral stimulation does (and that has nothing to do with penis size!).

Unfortunately, guys get self-conscious, too — just like you might worry about the size of your breasts, they worry about the size of their penises.

11. Why do guys sometimes not-so-subtly scratch their balls in public?

Guys sometimes need to itch or readjust because balls sometimes have a tendency to move uncomfortably in clothing or stick to the side of their leg, especially when exercising, walking, or sweating.

While this move might make a guy feel slightly more comfortable, it tends to make people around him feel kind of uncomfortable. Friedrichs' message to guys: "I'd recommend trying a little discretion if you're itchy or need to readjust!"

It's like dealing with a wedgie — it's more polite to wait till you're in private.