The digestive process really begins when you simply think about eating, or even when you smell the aroma of something you want. A drop in blood sugar, the time of day, and the nose also may combine to signal the body that it's time to eat.
Digestion is only simple at the general level.
As food enters the mouth, salivary glands mix special chemicals with each bite to make further digestion easier. Chewing your food well also gives the stomach and related organs time to generate particular kinds of chemicals adapted to what the tongue is tasting. For a normal person, what tastes or smells particularly good may be what the body has a strong need for at that time-fruits for quick energy, vegetables for calming endurance, and grains for protein to replace tissue or for more growth. As the body ages, the digestive system tires and slows down, especially under stress. Hydrochloric acid is needed to digest proteins, and is the only desirable inorganic acid found in the body.