The document outlines various causes for hard nipples, emphasizing that while they can be a normal occurrence due to stimuli like temperature changes or physical sensations, they can also indicate underlying health issues like infections, hormonal changes, and sensitivities. It details several factors contributing to hard nipples, including exercise, allergies, ovulation, and breastfeeding, as well as when to seek medical advice for symptoms indicating potential health concerns. Overall, the document aims to inform readers about both the benign and more serious causes of nipple hardness and discomfort.