The fat in Roux, coats the molecules of flour. That keeps the flour from absorbing liquid and the molecules take up a lot of space in the pot. This is the thickening action. The important thing to know is that the more you cook the Roux, the less time that you have between adding it to food and cooking that food, if you don't want the Roux to loose it's thickening properties. Flour just don't hold up for long. Also. If you DON'T cook the Roux enough (either on it's own or in the pot), then you will make the food that you are cooking taste like flour. Catastrophe. You just gotta get it right. This will help. Think of roux as being one of 4 types. BLOND BLANC D'OR BRUN. Blond roux is useful to thicken a soup or sauce that you want to keep white. You have to cook that sauce until the flour taste is gone. Then kill the heat and serve. Blond roux is also good for soup or sauce, and you can add it last minute. D'Ore (gold) roux is useful to thicken a stew. It's darker and will give you time to get it stirred in and served, before it starts to give out. It doesn't have to be cooked with the stew to get the flour taste out. Brun is a last minute roux. It won't hold up, but you do get a great dark color and a nutty flavor from the well cooked flour. To cook the roux. Melt butter in a pan and then add an equal quantity of flour as you stir. The final roux, will be crumbly when hot, and a hard ball when cold. Cook the flour until a patena or a finish comes to the roux. It looks like a powdery finish. This is BLOND. If you cook it a little longer the patina vanishes, it becomes a BLANC. When it starts to change color, it is useless for a white sauce as it changes color to gold. This is D'ORE. With a little more cooking it'll turn brown. This is BRUN. Always remove roux from the pan and let it cool a moment. Then add it to the food in the pot. A little goes a long way. You can store it in a ball tightly wrapped in plastic wrap.