Below is an instruction from Human. Write a response.
    ### Instruction:
    What is (are) Pain ?
    ### Response:
    Pain is a feeling triggered in the nervous system. Pain may be sharp or dull. It may come and go, or it may be constant. You may feel pain in one area of your body, such as your back, abdomen or chest or you may feel pain all over, such as when your muscles ache from the flu.
    