Nipples are weird and wonderful things; they go hard when you're turned on and stick out like sore thumbs when you're cold—and sometimes, just go hard for no apparent reason.
Only it turns out there is a reason behind the often unpredictable erections (bet you've heard that one before), and it's down to a highly specialized type of nerve cell that does nothing else except generate nipple erections. Yep.
According to the journal of Nature Neuroscience, hard nipples are normally involuntary and are controlled by the sympathetic nervous system, which also coordinates and regulates all our other unconscious bodily functions.
The study's co-author Alessando Furlan explained: "We've shown that the sympathetic system consists of many types of neuron that regulate specific functions in the body.
"For example, one thing we found was that goose bumps and nipple erections are caused by neurons that are specialized to regulate these functions by controlling the erectile muscles in these tissues."
Of course, being cold, breastfeeding, and sexual arousal can also prompt your nipples to stick out, which might be down to how sensitive the area is, or the smooth muscle cells around the areolae contracting when stimulated.
This article originally appeared on Cosmopolitan.co.uk. Minor edits have been made by the Cosmo.ph editors.