Whether you have one or not, you probably think you know everything there is to know about penises. But erectile dysfunction (ED) affects up to 30 million American men, many of whom don’t understand how or why it happens.

1. Most men with ED don’t actually have ED. Older men are more likely to suffer from erectile dissatisfaction than erectile dysfunction. This means that getting and keeping an erection has become more difficult and is more frustrating than it once was, but it’s not impossible.

2. Penises are fragile. Yes, you can break your penis. It’s not like a broken bone, though — instead, the blood vessels within the penis burst, causing painful swelling. According to the UK’s National Health Service, one-third of penile fracture cases are attributed to sexual intercourse where the partner is on top.

3. Erections happen early. Like in the womb, early. Male fetuses can experience erections in the womb.

4. Medications for ED don’t always work. Drugs designed to help men with ED regain a healthy sex life only work in about two-thirds of cases. Also, Psychology Today reports that some men need increasingly larger doses, which come with increasingly intense side effects.

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5. Shorter penises make bigger erections. Research shows that men with shorter penises experience erections that increase penis size by 86 percent. For longer penises, the growth is more like 47 percent.

6. The average erection is 5.6 inches long. A 2013 study attempted to settle penis size once and for all. The findings: The average erect penis is just shy of 5.6 inches. The data came directly from study participants, who self-measured, so there is the question of how many exaggerated.

7. The world’s largest erection is 13.5 inches. Jonah Falcon is said to have the world’s largest penis that, when erect, is well over 1 foot long. When flaccid, it measures 9.5 inches.

8. ED can signal a serious health problem. Sometimes, sexual problems are a sign of bigger issues. According to the Mayo Clinic, ED can be caused by heart disease, diabetes, Parkinson’s disease, multiple sclerosis, high cholesterol, and more.

9. Erections aren’t required for orgasm. Yes, men who can’t achieve erections may still have orgasms.

10. Lifestyle choices can make you more prone to ED. Alcoholism, drug use, being overweight, smoking, certain medications, and even “prolonged bicycling” can lead to ED, according to the Mayo Clinic.

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11. Men have between three to five erections per night. Every night while you’re sleeping, your penis is hard at work. The average man experiences up to five erections while they sleep, each lasting about 30 minutes.

12. ED stress can make matters worse. ED can cause psychological stress, and psychological stress can cause ED. When men are frustrated by their inability to perform, it can make it even more difficult to overcome ED. Talking with a doctor or sex therapist may help.