Why are some men "growers" and some men "show-ers?" and is there any way to change from one to the other? -- Stan, 23, Iowa

Any guy knows that Johnny’s size varies a lot — and not just due to an erection.

The penis is basically a temperature-sensitive sponge. Exposed to cold air or water, blood retreats into the body and any guy’s penis will shrink to pre-teen proportions. If you’re warm and relaxed, on the other hand, blood flow increases and the penis will expand in both girth and length while still being flaccid.

The average flaccid adult penis is between 3 and 3½ inches long in the warm, relaxed state. Obviously, some guys are longer or shorter than average. But the data show that shorter flaccid penises tend to enlarge more during erection than larger flaccid penises—so you can’t really predict how large a guy’s erection will be just by eyeballing his flaccid penis.

Dr. Harry Fisch is a board certified urologist. He’s here to answer reader questions in an effort to get guys to “man up about health.”