Recently, the topic of precocious puberty has attracted the attention of many parents, and many people take advantage of the summer holidays to bring their children to the hospital for consultation. Dongdong is a lively 6-year-old boy. One day when his mother gave him a bath, she noticed that his foreskin was very long and was surprised to find that she could not feel the two “little balls” unique to boys in his scrotum. I gave Dongdong a detailed physical examination and confirmed that his two testicles (balls) were located in the left and right inguinal canals with the help of a B-type ultrasound. Accordingly, I explained to Dongdong’s parents that this condition is medically known as “cryptorchidism”, which means that one or both testicles stop on their way down and do not enter the scrotum on the same side, and is a common variation in testicular position during male development. The incidence of cryptorchidism is reported to be about 30% in premature infants, 4% in newborns, 0.66% at the age of 1 year, and 0.3% in adults. The incidence decreases gradually during growth and development, indicating that the testis may still continue to descend after birth, and the chance of continued descent decreases significantly after 1 year of age. In normal humans, the testes should descend to the scrotum by the 28th to 35th week of embryonic development, which is of great importance in human evolution. This is because the scrotum provides the testes with the most comfortable temperature for spermatogenesis and development. The fleshy membrane layer under the scrotal skin can reflexively stretch and contract with the change of external temperature to regulate the temperature inside the scrotum, so the scrotal temperature in normal human is about 2℃ lower than the body temperature, and this temperature is necessary to ensure normal testicular development and normal spermatogenesis function after birth.