Below is an instruction from Human. Write a response.
    ### Instruction:
    What is (are) What I need to know about Kidney Stones ?
    ### Response:
    A kidney* stone is a solid piece of material that forms in a kidney when there are high levels of certain substances in the urine. These substances are normally found in the urine and do not cause problems at lower levels.
                
A stone may stay in the kidney or travel down the urinary tract.
    