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Cerebro-spinal fluid is the clear fluid that surrounds the brain and spinal cord.  It's primary function is to act as a shock absorber for the brain and spinal cord, but it also plays a role in preventing infection from entering the brain case or the spine.  In a normal person, the CSF is created and drained between 3 and 4 times a day. 

Because CSF is being continuously created, if it does not drain properly, it can quickly increase pressure in the skull and prevent the correct flow of blood to the brain. 

A lumbar puncture is a procedure where a surgeon draws CSF from the spinal column by inserting a syringe between two of the vertebrae.  The CSF can then be tested for a number of diseases.

Cerebrospinal fluid at Wikipedia