Below is an instruction from Human. Write a response.
    ### Instruction:
    What is (are) Cerebral Aneurysms ?
    ### Response:
    A cerebral aneurysm is a weak or thin spot on a blood vessel in the brain that balloons out and fills with blood. An aneurysm can press on a nerve or surrounding tissue, and also leak or burst, which lets blood spill into surrounding tissues (called a hemorrhage).
    