What makes Homemade Cranberry Sauce thicken properly?

The Homemade Cranberry Sauce thickens through gentle simmering rather than by adding a separate thickening agent. After the cranberries are combined with the sugar, orange juice, and water, the mixture is cooked uncovered over low heat. As it simmers, the cranberries burst and some of the liquid reduces, naturally creating the thicker texture described in the recipe.