Digestion is the act of absorbing nutrients from food in the process of breaking it (food) down into smaller molecules (may it be through chemical or physical means) in order for it to be able to release its nutrients.
Even before food enters the mouth, our glands in the mouth produce saliva. Mastication then appears which turns chewed food into a moist lump called “bolus.” The bolus has then traveled to the esophagus and reaches its next destination which is the stomach wherein the bolus is being broken down into simpler substances or more commonly known as catabolism with the help of the acid and enzymes produced by the stomach. Due to this process, the bolus has turned into a fluid-like chyme. The chyme moves down to the small intestine from the stomach and enters a region called the duodenum in which chemical digestion appears. Fats have become fatty acids, proteins into amino acids, and carbohydrates become glucose. The continuous backlash movement along the microvilli on intestinal lumen helps in absorption of all the nutrients released from the food. Finally, leftover water, fiber, and dead cells travel to the large intestine where the body drains the remaining fluid and a soft mass called “stool” remains and is then expelled when it is already time.
This is the whole gastrointestinal track and we can see how food changes its form in order for our body to absorb the nutrients and fluids that we consume.
Sources: https://www.bartleby.com/solution-answer/chapter-23-problem-1ilq-anatomy-and-physiology-1st-edition/9781938168130/by-clicking-on-this-link-httpopenstaxcollegeorglfooddigestion-you-can-watch-a-short-video/cad5bd3e-0e4c-11e9-9bb5-0ece094302b6