Below is an instruction from Human. Write a response.
    ### Instruction:
    What is (are) Kidney Stones in Adults ?
    ### Response:
    A kidney stone is a solid piece of material that forms in a kidney when substances that are normally found in the urine become highly concentrated. A stone may stay in the kidney or travel down the urinary tract. Kidney stones vary in size. A small stone may pass on its own, causing little or no pain.
    