your breast may be developing. if it is very painful, possibly something else.
then see a doctor.
You're going through puberty.Dont worry it's normal for this to happen.I've had that happen to me.
man i have the same thing but I'm only 12 and its in my left nipple..my dad says its puberty but i don't think it is
They also "secrete" (for lack of a better word) breast milk.
Stimulate it.
No
yes they will be hard
Your eather horny or cold!
Most likely it's a cyst.
So hard it hurts!
You get cold, or stimulate them mentally or physically.
Guys LOVE Hard Nipples. It Is Easier For Them To Suck On And Wrap Their Tongues Around!
About as harmful as getting a hard nipple.