Fish are aquatic vertebrate animals that are typically ectothermic (previously cold-blooded), covered with scales, and equipped with two sets of paired fins and several unpaired fins. Fish are abundant in the sea and in fresh water, with species being known from mountain streams (e.g., char and gudgeon) as well as in the deepest depths of the ocean (e.g., gulpers and anglerfish). Stock is a flavoured liquid. It forms the basis of many dishes, particularly soups and sauces. Stock is prepared by simmering various ingredients in water, including some or all of the following: Meat Leftover cooked meat, such as that remaining on poultry carcasses, is often used along with the bones of the bird or joint. Fresh meat makes a superior stock and cuts rich in connective tissue such as shin or shoulder of beef or veal are commonly recommended, either alone or added in lower proportions to the remains of cooked poultry to provide a richer and fresher-tasting stock. Quantities recommended are invariably in the ratio of 1 part fresh meat to 2 parts water. Pork is considered unsuitable for stock due to its greasiness (although 19th century recipes for consomme and traditional aspic invariably included slices of mild ham) and mutton was traditionally avoided due to the difficulty of avoiding the strong tallowy taint imparted from the fat. Bones Veal, beef, and chicken bones are most commonly used. The flavour of the stock comes from the cartilage and connective tissue in the bones. Connective tissue has collagen in it, which gets converted into gelatin that thickens the liquid. Stock made from bones needs to be simmered for longer than stock made from meat (often referred to as broth). Pressure cooking methods shorten the time necessary to extract the flavour from the bones. Mirepoix A combination of onions, carrots, celery, and sometimes other vegetables. Often the less desirable parts of the vegetables (such as carrot skins and celery ends) are used since they will not be eaten. Herbs and spices The herbs and spices used depend on availability and local traditions. In classical cuisine, the use of a bouquet garni (or bundle of herbs) consisting of parsley, bay leaves, a sprig of thyme, and possibly other herbs, is common. This is often wrapped in a cheesecloth "bag" and tied with string to make it easier to remove it once the stock is cooked. Broth is very similar to stock, and often the terms are used interchangeably. Usually, broth refers to finished product, while stock is used as an ingredient (thus stock may become broth). Other times, broth is used to refer to a liquid made in the same way as stock, but meat is substituted for bones. However, with some stock/broth made from vegetables and some made from both bones and meat, this cannot be considered a hard-and-fast rule. Today, ready-made stock and stock cubes consisting of dried, compressed stock ingredients are readily available. These are commonly known as bouillon cubes (or oxo cubes, after a common brand of stock cube sold in Britain) or cooking base.