The largest and one of the most important organs, liver has immense nutritional value . . . providing, that is, that it comes from a fairly young animal. Because liver acts as a clearinghouse for substances that enter the body, it tends to store and absorb unwanted chemicals, medicines and hormones that an animal might be fed. Naturally, the older the animal the greater the accumulation of these unwanted substances, which, according to some, offset liver's nutritional value. For this very reason, many people choose the more expensive calf's liver over beef liver. There are several ways to distinguish between the two. The color of beef liver is reddish-brown, compared to the paler pinkish-brown of calf's liver. Liver from a mature animal also has a stronger odor and flavor than that from a youngster. Additionally, it will be less tender. Besides beef and calf's, the most common animal livers used in cookery are lamb, pork, poultry and goose, the latter used mainly to produce pâté de foie gras. The strongest-flavored and least tender of the livers is pork, while poultry livers are the most mild and tender of the lot. All livers are usually available fresh — beef and chicken livers may also be purchased frozen (though the quality of frozen liver is considerably lower than that of fresh). While chicken livers are sold whole, most of those from other animals are marketed sliced. Look for liver that has a bright color and moist (not slick) surface. It should have a fresh, clean smell. Refrigerate loosely wrapped for no more than a day. Liver can be prepared in a variety of ways though quick sautéing is the most popular. It toughens quickly with overcooking. Liver is rich in iron, protein and vitamin A.