Some attentive mothers will find that when moving the baby’s knee joint, there is a noticeable ringing sound and sometimes a bouncing sensation, but at this time the baby does not seem to have any pain or discomfort. The meniscus is a structure of two cartilage pads in the knee joint that stabilizes the joint and cushions the pressure. The meniscus is surrounded by ligaments that restrict movement, and when the knee joint is flexed and extended, the meniscus slides rapidly in the joint to accommodate the change in joint position. Because the ligaments of young infants are weak and lax, the meniscus is very mobile and slides over the joint surface of the knee to a certain degree, resulting in a “popping sound”. As they get older, the ligaments become thicker and the structure of the knee joint is gradually altered, and this joint popping noise disappears. Therefore, in most cases, babies with knee rattles should not worry. In a small number of children, the meniscus has an abnormal shape, with thick edges and a depressed center that is closer to a circular shape, so it is called a “discoid meniscus”, which is more likely to cause joint popping. The diagnosis can be confirmed by MRI of the joint. If the “disc-shaped meniscus” is really clear, it usually does not affect children’s walking and normal activities, but when they grow up, they need to be careful when playing soccer and other strenuous sports, otherwise, the meniscus will be easily damaged. If your baby has a joint popping sound in the hip joint, you must consider the possibility of congenital hip dislocation. Once you find hip popping sounds, you should go to a pediatric orthopedic specialist for examination without delay.