Soups and sauces can be thickened in a variety of ways. A sauce must the thick enough to cling to food, but not so thick that stands up for himself. 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. Source: A roux should be cooked for a short period of time to finish the sauce or soup does not taste of starch in the flour. If cooked for a very short period of time, is called a blond roux. If cooked longer until it acquires a brown color, is called a brown roux. The favorite in the kitchen Roux is made by mixing melted butter and flour. Many cooks clarify the butter first because the liquid in whole butter tends to gelatinize some of the starch and make the roux difficult to work with them. A roux made with butter gives a nice rich flavor to sauces and is easy to work. Margarine and oils can be used to make a roux as well, but due to the lack of flavor that is very rarely the first option.