The nipples of men and women can also become erect. Women's nipples become erect when they are breastfeeding (giving milk to their babies from their breasts). Men and women may also have an erection of their nipples when they are sexually excited, or if they feel cold. Nipples do not have any tissue in them that fills with blood. Instead, the body's nervous system makes muscles in the nipples contract (become shorter), causing them to stand up and become slightly harder.