Below is an instruction from Human. Write a response.
    ### Instruction:
    What is (are) Kidney Stones ?
    ### Response:
    A kidney stone is a solid piece of material that forms in the kidney from substances in the urine. It may be as small as a grain of sand or as large as a pearl. Most kidney stones pass out of the body without help from a doctor. But sometimes a stone will not go away.
    