Define Principle of fehling test - reduction tests?

Sugars that possess a free or potentially free (those that can be converted to free) aldehyde or ketonic group have a property of easily reducing metal ions such as copper, iron, mercury, bismuth etc. The alkali from Fehling's reagent acts on sugar molecules and converts them to enediols. These enediols are unstable and highly reactive and thus reduce cupric ions to cuprous ions. These cuprous ions combine with the hydroxyl ions to form cuprous hydroxide, which, on heating, forms a red precipitate of cuprous oxide.