THINGS YOU DIDN'T KNOW ABOUT YOUR PENIS

1. Use It or Lose It

You need to have erections regularly to keep your penis in shape. "It has to be essentially exercised," says Tobias Kohler, MD, assistant professor of urology at Southern Illinois University School of Medicine.

To maintain a healthy tone, the smooth muscle of the penis must be periodically enriched with oxygen by the rush of blood that engorges the penis and makes it erect, Kohler says.

If a guy is physically able to get erect, but never has erections during the day -- maybe he finds himself in very un-erotic circumstances for a long time -- he needn't worry. The brain has an automatic penis maintenance function built in.

Impulses from the brain cause erections during the dreaming phase of sleep, called the REM phase. It doesn't matter if you're having a hot sexdream or a zombie apocalypse nightmare -- your penis gets hard during that period of the sleep cycle.

But some men are physically unable to get erections, such as those who've suffered trauma to the nerves involved or who have nerve or blood vessel damage caused by diabetes.

"If they don't do anything to maintain normal erections, they will get shortening of the penis," Kohler says. Without regular erections, penile tissue can become less elastic and shrink, making the penis 1-2 centimeters shorter.

A device like a vacuum pump, which forces the penis to swell with blood, can help men with physical erection problems maintain a healthy penis, Kohler says.

2. Your Penis May Be a 'Grower' or a 'Show-er'

Among men, there is no consistent relationship between the size of the flaccid penis and its full erect length.

In one study of 80 men, researchers found that increases from flaccid to erect lengths ranged widely, from less than a quarter-inch to 3.5 inches longer.

Whatever the clinical significance of these data may be, the locker-room significance is considerable. You can't assume that a dude with a big, limp penis gets much bigger with an erection. And the guy whose penis looks tiny might get a surprisingly big erection.