1, because all the food you eat is processed in your mouth first. Hard teeth are like a clever grinder, cutting and grinding, turning large pieces of food into small particles; At the same time, the salivary glands keep secreting saliva, and the tongue keeps stirring food, so that food and saliva are fully mixed. After mixing evenly, amylase in saliva can react with starch in food to convert starch into maltose, which is convenient for further digestion and absorption in gastrointestinal tract. Also, when the tongue is stirring food, the taste of food can stimulate the taste nerve on the tongue. The taste nerve immediately reports to the brain. After receiving the report, the brain immediately orders the stomach and pancreas to produce digestive juice and prepare to receive food.