Is it normal for a 13 year old boy to have knot in his nipple?

It is not uncommon for boys to develop a small, hard lump or knot under the nipple during puberty. This is usually a benign condition called a "gynecomastia". It occurs due to the temporary increase in estrogen levels in the body during puberty, which can cause breast tissue to develop in males. In most cases, gynecomastia resolves spontaneously within a few months or years. However, if the lump is causing pain or discomfort, or if it persists for a long time, it is recommended to consult a doctor for further evaluation.