system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: Why does a penis shrink in cold water?

I know this may sound funny to some and immature but I've always wondered why. 

(Update)
Our human body is composed of water for the majority, and when water freezes in expands. How come it shrinks when it's in cold water?
gpt
Penis size is regulated by blood flow into the penis.  The mechanism behind an erection, for example, is pretty much just an increase in blood flow to that part of the body.  This causes the penis to expand much like a balloon when it's filled with water.  In other words, there is a relationship between blood flow and penis size.

Heat regulates blood flow.  When the temperature is low, the body undergoes vasoconstriction, the constriction of the tubes that carry blood throughout the body.  By making these tubes smaller, less heat is exchanged between the blood and the outside environment, so the blood stays warm.  However, vasoconstriction also decreases the blood flow to the penis, which as explained in the first paragraph, affects size.