IV. DIGESTIVE SYSTEM
The digestive system converts the food you eat into soluble particles for the body parts to absorb and use. The digestive process starts when the food enters the mouth, it is ground by the teeth and reacts with the chemicals in the saliva. The process of swallowing takes place next. A flap called the epiglottis closes the windpipe (trachea) to prevent food from entering the lungs and  forces the food to enter the esophagus. The food is pushed down the esophagus by peristalsis (contractions of the  involuntary muscles along the esophageal wall). 
These automatic movements are maintained all along the 9 m long alimentary canal, which is composed of the mouth, esophagus, stomach, small intestine, large intestine and rectum. Food particles are constantly mixed with the digestive acid in the stomach for about 3 to 5 hours; this acid is so powerful that it can burn holes on your clothes! (If you feel hungry but resist to eat, your empty stomach will be harmed by the acid because there are no food particles for the acid to mix with.) As the food particles become porridge-like, it leaks out slowly to the small intestine, where the next stage of digestion takes place.