Below is an instruction from Human. Write a response.
    ### Instruction:
    What is (are) Ovarian Cysts ?
    ### Response:
    A cyst is a fluid-filled sac. In most cases a cyst on the ovary does no harm and goes away by itself. Most women have them sometime during their lives. Cysts are rarely cancerous in women under 50. Cysts sometimes hurt - but not always. Often, a woman finds out about a cyst when she has a pelvic exam.
    