Roux is a mixture of butter and flour cooked. It is generally added uncooked to thicken a sauce or a soup, but the flavor is much better if it is first cooked, and the sauce or soup added to it. When the butter first comes to a boil in a small stew-pan the sifted flour is sprinkled in, and both are mixed well together over the fire with an egg whisk until the flour is well cooked. A part of the sauce or soup is then stirred in until it becomes smooth and thin enough to add to the main sauce or soup. If the roux is intended for a white sauce it is not allowed to color; if for a brown sauce it may be colored, or brown flour may be used.