system
Given the following post, respond as if you're commenting on Reddit:
human
Why do peoples nipples get hard when its cold outside?

I don't know how much more I could elaborate on this.
gpt
They are trying to preserve heat. 

When a nipple is 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. 

Decreasing surface area is the ultimate reason why nipples get hard in the cold. It is not caused by erectile muscle, but rather by simple contraction of smooth muscle. The process is most similar to that of a hair follicle standing up on end...