Soups and sauces can be thickened in a variety of ways. A sauce must the thick enough to cling to the food, but not so thick it stands up on its own. Starches are by far the most common thickening agent. Cornstarch, arrowroot, waxy maize and the ever popular, roux (roo). But what is a roux and how does it work?Roux is a cooked mixture of equal parts by weight of fat and flour. If you mix a starch with water, such as cornstarch it is called a slurryHow does it work?Starches thicken by absorbing w.