Sometimes nipples are hard, sometimes they are not. Whatever they are, it seems we have absolutely no control over nipples.
乳頭有時會變硬，有時不會。不論哪一種情況，我們似乎完全沒辦法控制乳頭。
It is well-known physiological mechanism that, during chilly weather, our skin would tighten and contract, causing nipples to shrink and harden. It’s to reduce the surface area of the nipple, and therefore decrease the amount of energy required to keep it warm.
我們的皮膚在寒冷天氣下會緊繃、收縮，乳頭也會跟著縮小、變硬，這是眾所周知的生理機制。這個變化是要縮小乳頭的表面積，進而減少維持溫暖所需的能量消耗。
And sure, we also appreciate nipples’ function in sexual activity. They react in the same way as other parts of the body. But a new study has found that nipples sometimes go hard for no reason whatsoever. They seem to have a mind of their own.
當然，從事性行為時，我們也會察覺乳頭的變化，它們的反應與身體其他部位無異。然而，一項新研究發現，乳頭有時也會沒來由地變硬，就像會自己思考似的。
Researchers at the Karolinska Institutet in Sweden have found that nipples have their very own "special nerve" that causes goose bumps and nipple erection. The study, published in the journal Nature Neuroscience, explains that the sympathetic nerve system comprises different neurons that regulate specific physiological functions.
瑞典「卡羅琳學院」研究人員發現，乳頭有自己的「獨特神經」，會產生雞皮疙瘩、乳頭勃起等反應。這項發表於「自然神經科學」期刊的研究解釋，交感神經系統是由不同神經細胞所組成，不同的神經細胞則會引發特定的生理反應。
Basically, even when we’re not aroused, neurons that regulate nipple erection will function involuntarily – and without "conscious control".
基本上，即使沒有受到挑逗，控制乳頭勃起的神經細胞也會本能地運作，且「不受意識控制」。
Source article: http://iservice.ltn.com.tw/Service/english/english.php?engno=1074188&day=2017-01-26