You have grown up together and shared many experiences, but how well do you really know your penis? Here are 5 penis facts you probably did not know.

You can break your penis If the penis is violently twisted when erect, it can break. There are no bones in the penis, but the tubes that fill with blood during an erection can burst. Blood pours out of them inside the penis and causes a very painful swelling. Reported cases of penile fracture are rare, but it's thought that some men are too embarrassed to report it to their doctor. Damage during sex, where their partner is on top, is responsible for about one-third of all cases. The breakage usually occurs when a man's penis slips out of his partner and is violently bent.

Men have several night-time erections On average, a healthy man has 3 to 5 erections during a full night's sleep, with each erection lasting 25 to 35 minutes. It's common for men to wake up with an erection, informally called a "morning glory". This is in fact the last in the series of night-time erections. The cause of night-time erections isn't fully understood. But studies suggest they're closely associated with the phase of sleep known as rapid eye movement (REM) sleep. This is when dreaming is most common. Whatever their cause, most doctors agree that night-time erections are a sign that everything is in working order.

Small penises make big erections Shorter penises increase more in length than longer ones when they become erect. Research based on the penis measurements of 2,770 men found that shorter penises increased by 86% when erect, nearly twice that of longer penises (47%). In the 1988 study published in the Journal of Sex Research, researchers also found that the difference in length between a short penis and a longer one was a lot less obvious when erect than when flaccid. For example, the flaccid penises varied in length by 3.1cm (1.2 inches), whereas the average erect lengths differed by only 1.7cm (0.67 inches).