This is because the saliva changes some of the starch of the food into sugar.
~4.~ After we have chewed the food, we swallow it, and it passes down through the oesophagus into the stomach.
~5.

Stomach Digestion.~--As soon as the morsel of food enters the stomach, the gastric juice begins to flow out of the little glands in which it is formed.

This mingles with the food and digests another portion which the saliva has not acted upon.

While this is being done, the stomach keeps working the food much as a baker kneads dough.

This is done to mix the gastric juice with the food.
~6.~ After an hour or two the stomach squeezes the food so hard that a little of it, which has been digested by the gastric juice and the saliva, escapes through the lower opening, the pylorus, of which we have already learned.