A cyst is a closed lesion on the surface of the skin, typically filled with pus or other bodily fluids. Skin cysts grow very slowly, and are typically painless to the touch and smooth on the surface. Common causes of cysts include clogged glands, the presence of foreign objects such as earrings or body piercings, or the byproduct of an infection. In the vast majority of cases, cysts will resolve on their own without medical intervention. However, in the case of abnormally large cysts, or cysts which are at risk of rupturing, they can be surgically removed. Some cysts also respond well to cortisone injections, which shrink their size and relieve secondary symptoms.