Digestion is the process of metabolism where by a biological entity processes a substance in order to chemically and mechanically convert the substance for the body to use. In mammals, preparation for digestion begins with the cephalic phase in which saliva is produced in the mouth and digestive enzymes are produced in the stomach. Mechanical and chemical digestion begin in the mouth where food is chewed, and mixed with saliva to break down starches. The stomach continues to break food down mechanically and chemically through the churning of the stomach and mixing with enzymes. Absorption occurs in the stomach and gastrointestinal tract, and the process finishes with excretion.