We all develop nipples at the same time
When in utero, the nipples are developed before all sex organs. Human embryos fundamentally start out with the same XY chromosomes. In biological males the SRY gene will activate on the Y chromosome some weeks after conception. However, before that happens, breast tissue begins to develop and that comes with nipple tissue, too. So, before you have a vagina or a penis, you have what will grow into nipples. That's why men have them too.
Nipples are unique, like fingerprints
No two nipples look the same. Like your fingerprints, they have their own tiny indentations, raised bumps, colours, and other physical traits. A person can even have two nipples that don't look the same as each other. They can have asymmetrical nipples and they can have more than two of them (hello Chandler Bing!). In fact, in 2012 a man was discovered with seven.
You can have a nipple-gasm
Both women and men have nipples that are sensitive to touch and are erogenous (which means they can derive sexual pleasure). The extent of sensitivity is dependent person-to-person: some people (of both sexes) can experience orgasm strictly through nipple stimulation and no other touching, while touching the nipple does little (or seemingly nothing) for others.
They're hairy
Almost every mammal grows some sort of hair around the nipple. Even if it's just a couple of little hairs. This is completely normal, even for women. Lots of men will let the hair around their nipples grow naturally, but equally, many like to groom them too. Whatever your gender, nipple hair is natural and you're free to remove, or keep it, as you wish.
Nipples can bleed
When you watch people running marathons, you'll sometimes see them with tape on their nipples. The combination of repetitive friction and sweat will chafe the nipples so much the skin might actually break. This will cause bleeding, but isn't exclusive to long-distance runners – you might experience chafing/bleeding in many daily situations where you're sweating and your nipples are rubbing against fabric, especially inorganic material.
Changes to your nipples can be signs of breast cancer
Nipples come in all shapes, tones, colours, and sizes. We are told to associate lumps in the actual breast with cancer, but changes to a nipple itself (e.g. hardening, scabbing, inversion) signals it's time to get a mammogram. This includes men if they see nipple changes – men get breast cancer too and about one per cent of cases are in males.
They have sweat glands
Not the nipple itself, but rather the areola around it, has sweat glands. That's right, sweaty nipples are a thing. The biological purpose for this is to secrete fluid for lubrication during breastfeeding, and this sweat actually produces a scent that attracts a baby to it's mother's nipple.
Breast implants can change nipple sensitivity
After any kind of breast augmentation, be it an enlargement, lift, or reduction, you run the risk of losing some or all nipple sensitivity. Conversely, there's also a chance the nipples will become more sensitive – to the point of pain. Both can be temporary or permanent, and complete nipple desensitisation is most common in large implants where the skin that runs over the nerves has been overstretched.
If you want to pierce a nipple, be sure
Nipple piercings take six months or longer to heal – it's not the same as the skin in your earlobe. Nipple piercings will close up extremely fast if the piercing comes out. Your nipples can be made permanently larger with piercings, and they will often deform in shape when you take the piercing out – they'll never look natural again, so before piercing a nipple or two, make sure you're prepared for all of this.
