It's irritating and downright embarrassing when our nipples poke themselves through a t-shirt, sweater, or even a padded bra. These “hard-ons” are most common during cold weather, sexual arousal, and breastfeeding for moms, but how and why do they get erect? What's the point of the point?

In the DNews video, "Why Do Nipples Get Hard?" host Julian Huguet explains researchers have identified eight different types of nerves that carry messages for the sympathetic nervous system, which is part of the body's neural wiring that makes the heart race, palms sweat in anxious moments, produce goosebumps, constrict blood vessels, and harden nipples in cold weather. There's a specific type of neuron that exists just to make the nipples erect. So, when these neurons are activated by the neurotransmitter norepinephrine, they cause muscles to contract, which tightens the areola and causes the nipples to harden.

Do nipples have an evolutionary purpose?

The most obvious reason for hard nipples is for breastfeeding, as a baby's suckling reflex is activated when something strikes the roof of its mouth. Typically, a stiff nipple is what helps an infant latch on for a meal. A mother's breast milk will provide food to her baby via the nipple.

Aside from breastfeeding, nipples get hard when it's cold outside or during sexual arousal. Researchers hypothesize we get "nipply" when it's "nippy" because the muscles in our areolas are under hair follicles that respond to the same stimuli. Those who discovered the "nipple neuron" also found another neuron that responds to norepinephrine, which causes our hair to stand on end. The areola have smooth muscle cells that contract when stimulated; cold weather can make the skin pucker inward while the nipples stick out.

This could be a vestigial trait — a remnant of a trait that used to exist in our ancestors — that led their hair to puff up to trap air and keep warm. It's also frequently suggested our hair rises in stressful situations in an effort to make us look bigger, sort of like a cat puffing out its tail.

In the role of sexual arousal, the erect nipple still remains a mystery. It's one of the elements of the "arousal" stage of the sexual response cycle, which includes desire, arousal, orgasm, and resolution. Nipple erections are also described as one of the "non-genital peripheral mechanisms," according to Boston University School of Medicine.

We're not exactly sure what's going on with our nipples. It's a highly sensitive area shrouded in mystery.

So, if the nipples are out, chances are you’re either breastfeeding, cold,  sexually aroused, or reacting to something in your body that science isn't aware of — yet.