If you’ve ever dipped your hand in ice cold water or stepped outside in extremely cold temperatures, chances are your penis has shriveled up and “disappeared.” This common phenomenon, known as penis shrinkage, is the body’s fight-or-flight response for when you’re cold, but how and why does it happen?

Just like the penis grows during an erection, it can sometimes shrink. The flaccid penis varies in size and blood supply within any given man. Exposure to cold temperature, either from water or air, affects blood circulation to the penis. This leads to a narrowing of the blood vessels, known as vasoconstriction, according to Harvard Health Publications.

Vasoconstriction causes the corpora cavernosa — the spongy erectile chambers — to become less engorged with blood, making them smaller. Meanwhile, blood flow is dispersing to other organs in the body that may need it, as the body enters survival mode. In an episode of The Doctors, Dr. Aaron Spitz, an urologist, explains penis shrinkage is the body's way of protecting you when you're under stress.

“We have these nerves called sympathetic nerves, which respond to stress and they protect us when we're under stress. Cold is a stress and when our body feels cold those nerves kick in shunting all that blood out of our fingers, our toes, our penis," he says.

But fear not: The penis will still be able to expand to its usual size once the blood flow begins again.