Below is an instruction from Human. Write a response.
    ### Instruction:
    What causes Gallstones ?
    ### Response:
    Imbalances in the substances that make up bile cause gallstones. Gallstones may form if bile contains too much cholesterol, too much bilirubin, or not enough bile salts. Scientists do not fully understand why these imbalances occur. Gallstones also may form if the gallbladder does not empty completely or often enough.
    