Cerebrospinal fluid
Head injuries and some diseases can make too much CSF build up in the brain.  This is called hydrocephalus.  The extra fluid puts pressure on (squeezes) the brain.  This is called increased intracranial pressure.  If the pressure gets high enough, the brain's blood vessels get crushed, and blood cannot get to the brain.  If a person's brain does not get enough blood and oxygen, they will become unconscious and their brain cells will die.  Eventually, the person will die.