To skim is to remove the layer of fat, foam, or impurities that rises to the surface of a liquid, such as stock, milk, or a simmering sauce, usually with a spoon or ladle. Skimming stock or broth produces a clearer, less greasy liquid; skimming milk separates out the cream, leaving skim (nonfat) milk behind. Fat can also be skimmed off after chilling, once it solidifies on the surface, or removed with a fat separator. The technique is central to clarifying stocks and consommés and to making leaner sauces and gravies.