system
Given the following post, respond as if you're commenting on Reddit:
human
Why do our nipples get hard when we're cold?
gpt
When exposed to cold temperature skin reflexively contracts, causing the nipple to become smaller, wrinkled and "hard." The ultimate function of this reflex is to decrease the amount of surface area on the nipple that is exposed to the cold. This decreases the amount of body heat that is lost to the cold and ultimately the amount of energy it takes to sustain homestasis (keeping normal body temperature) in an organism. So decreasing surface area is the ultimate reason why nipples get hard in the cold. It's the same thing as goosebumps on other parts of your skin.