Soup is a liquid food that is made by combining ingredients, such as meat, vegetables or legumes in stock or hot water, until the flavor is extracted, forming a broth. Boiling was not a common cooking technique until the invention of waterproof containers (which probably came in the form of pouches made of clay or animal skin) about 5,000 years ago.
The word "soup" comes from the older word "sop," meaning a piece of bread served with some type of liquid. One of the first types of soups can be dated to about 6000 B.C., earlier than other records, with the main ingredient being hippopotamus.
Over the centuries, the terms gruel and potage have become separated from broth and stock (and their refinement, consommé). The language may have shifted over time, but the modern definitions of soup and stew were established in the 18th century: soups usually are more liquid; stews are thicker, containing more solid ingredients. Stews are cooked in covered containers for longer periods of time, at a gentle boil with less water and at a lower heat.
Traditionally, soups are classified into two broad groups: clear soups and thick soups. The established French classifications of clear soups are bouillon and consommé. Thick soups are classified depending upon the type of thickening agent used: purées are vegetable soups thickened with starch; bisques are made from puréed shellfish thickened with cream; cream soups are thickened with béchamel sauce; and veloutés are thickened with eggs, butter and cream. Other ingredients commonly used to thicken soups and broths include rice, flour, and grain.