How is it produced and where does it come from?
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Saliva is a water-based proteinaceous substance that is produced by our salivary glands under the tongue. It contains an enzyme called amylase that is designed to break down starches into sugars so that we can digest starches more easily.

Try this experiment to show what I mean. Chew a Saltine cracker and keep it in your mouth for a few minutes. Notice that initially, you're tasting the starch of the flour in the cracker. After a few minutes, the starch starts breaking down into simpler sugars and the cracker starts tasting sweeter. You're tasting the breakdown of the starch caused by the amylase enzyme.