To live we must feed ourselves. The body, though, cannot use food as it is when eaten.
Teeth do cut and chew food in order to reduce it to gruel. It then descends through the oesophagus down to the stomach.
Here the mashing continues for hours, until the gruel grows liquid. Stomach muscles work as a mixer.
Why do we feel hungry?
Stomach is a big elastic pocket. Once full you are not hungry anymore. When empty you do. In the morning, being the pocket empty, you are normally hungry. You need a consistent breakfast. During the morning, the stomach works and empties out, and when it is lunchtime...

Intestine
After the stomach, food, reduced in a liquid gruel, goes through the intestine, where it is separated.
The intestine is a very long channel going from the stomach to the anus.
1) Small intestine.
2) Large intestine
3) When such little appendix infects, you feel a strong stomach-ache. An operation is necessary for the extraction, though it is nothing to worry much about.

Small intestine separates what is good for our organism, and what is not. What is, crosses the intestine and goes in the blood. What is not goes to the large intestine, to be expelled trough the anus... excrement, at last.

Stomach-ache.
The stomach is a fragile organ. If we are too demanding, and commission too much work, he suffers and complains.
When we eat too much, stomach cannot do the job, and we throw up.
Stomach-ache is even possible when over-munching in between meals, while playing, for instance.