Answer:
A cyst is a membrane sac which often encloses something within the sac. For example, a cyst may be hollow or may be filled with fluid. Another type of cyst may contain tissue or other types of cells. A cyst forms for a wide range of reasons but one of the most common reasons is that a cyst is the body's attempt to enclose and literally "wall off" something that could cause illness, infection, or another problem. For example, in some pregnancies, the embryo does not form correctly and the body will "wall off" and enclose the embyronic sac within a "cyst". That type of cyst is surgically removed and when it is sent to the lab, the technicians actually find parts of an embryo or embryonic cells within the cyst. In another less dramatic example, some people will get a small moveable cyst on a knuckle which can cause pain if it is resting on a nerve. This kind of cyst is most often simply full of fluid.