Touching the nipple and the nipple will become hard is usually a normal physiological phenomenon, mostly caused by the sensitivity of the nerve endings in the nipple area. There are a lot of neurons on the nipple, thus the nipple is more sensitive. When touching the nipple, especially when touched by the opposite sex, the sensitive nerves of the nipple will transmit signals to the brain, and the brain will transmit excitation signals to the smooth muscle under the nipple and areola, causing the smooth muscle to contract, resulting in nipple hardening. Touching the nipple, the nipple will harden, this process is usually a manifestation of sexual arousal, also known as nipple erection, and the male external genitalia in the time of sexual arousal erection is similar to, mostly belongs to the normal physiological reflex.