Do an experiment to see how. Chew on a plain soda cracker. At first it doesn't taste sweet at all. Get it good and gooey and hold it in your mouth. After a while it will taste sweet.

Here's why. Your saliva has a chemical in it that changes the starch in the cracker into sugar. Sugar goes directly into your bloodstream where its energy is used by your body. But starchy foods have to be digested -- broken down into sugar -- before they can enter your blood. This digestion starts in your mouth when you chew starchy food and mix it well with saliva. Usually you swallow the cracker before you give yourself enough time to tast the change.