The brain is the prime actor for what the body does. The brain decides what the body needs to do in every little second of it's life. When the body needs to drink, the brain tells the arm to pick up the glass and put it to the mouth, then tells the gag-reflex to not activate, and for the throat to push the water down to the stomach. Then the brain sends commands to the stomach to digest the water and pull out all the necessary nutrients needed. THEN the brain sends commands to dispose of the wasted parts of the water, which comes out in pee form.
