system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: Why do nipples get hard in the cold?
gpt
First comment is shadow banned.

Anyways, all skin tightens when it is cold.  That is what goosebumps are, the skin tightening and hair follicles standing out.  Nipples, however, are much more supple due to thinner layers and not bound to what is underneath, so there it is easier to scrunch up.

Scrotum and labia majora are similar in this regard.