Perky nipples are a sign of fertility and desire. It’s like they’re saying, “Hey! Check out how easy I’ll be able to breastfeed a baby after you get me pregnant! Do me!”
When our ancestors got cold they would get goosebumps to raise their fur up, which created a fluffy layer of warmth. Most of us are pretty much fur-free but the ability to get goosebumps––including hard nipples–– still remains.
Also, the areolae (the colored areas surrounding the nipples) have smooth muscle cells that contract when stimulated. Cold weather, or stimulation, can make those muscles tighten, causing the skin pucker inward while the nipples stick out, just like the rest of our muscles. Brrr!!!
Girl’s nipples get hard because they really, really want you to try to Google that question. No, REALLY. That’s why!