Below is an instruction from Human. Write a response.
    ### Instruction:
    What is (are) Bipolar Disorder ?
    ### Response:
    Bipolar disorder is a serious mental illness. People who have it go through unusual mood changes. They go from very happy, "up," and active to very sad and hopeless, "down," and inactive, and then back again. They often have normal moods in between. The up feeling is called mania.
    