Grab the one of the middle knuckles of one of your fingers firmly with your other hand. Now slide the skin to your fingertip, then down to the base knuckle.
What's that? You can't do that, because the skin is fixed in place? Well, imagine my surprise when I learned penises aren't supposed to be like that.
There's a whole range of procedures that fall under the umbrella term of circumcision, depending on how much they cut off. It goes from snipping off a little ring at the tip (the traditional bris), to outright removal of the entire mobile skin system. I figure I've lost at least 15 square inches (an index card size) of adult tissue.
its called phismosis, i believe. it can constrict your penis to the point to "strangle it" an untreated one can accumulated "smegma" and it theorized it poses a higher risk of penile malignancy, which is rare. theres a similar called paraphismosis, which can caused by inflammation.