Below is an instruction from Human. Write a response.
    ### Instruction:
    What is (are) Stroke ?
    ### Response:
    A stroke is a medical emergency. Strokes happen when blood flow to your brain stops. Within minutes, brain cells begin to die. There are two kinds of stroke. The more common kind, called ischemic stroke, is caused by a blood clot that blocks or plugs a blood vessel in the brain.
    