system
Given the following post, respond as if you're commenting on Reddit:
human
Why do our nipples get hard when it's cold?
gpt
Your skin in general gets tighter (contracts, less area) to conserve heat. hence the "goosebumps" of the skin getting so tight you notice the hair follicles protruding as bumps.

When the skin around your nipples gets tight the nipple gets firmer.