There are two main reasons for the unexplained bulge in the knee joint. This is due to an obstruction in the secretion process of the sebaceous glands, resulting in a buildup of secretions, poor excretion, and local enlargement, which produces a large amount of beanbag-like material. In the beginning, there is no obvious pain in the process, but as the volume increases, it will stimulate and squeeze the peripheral nerves and cause pain, so it feels like it appears for no reason. Generally, surgical measures must be taken to completely clean out the sebaceous cyst’s envelope and the bean-like material inside, and then suture it to be able to cure it, and once it becomes infected, it needs to be surgically incised and drained. In the second case, the knee joint may have been bitten by mosquitoes and insects, resulting in a local allergic reaction and a red and swollen bag.