Sausages are typically packed into casings, which are the material that encloses the filling of a sausage. There are two types of casings: natural and artificial. Natural casings are made from animal intestines or skin, while artificial casings are made of collagen, cellulose, or plastic. Natural casings are the traditional choice and are made from the sub-mucosa of the small intestine of meat animals, such as pigs, sheep, goats, cattle, and sometimes horses. They are flexible, tender, easy to stuff, and are durable enough to hold up to smokehouse processing. Artificial casings, introduced in the early 20th century, are usually more tender than natural casings and provide better weight and size control of the sausage. Collagen casings are the most common type of artificial casing and are produced from animal collagen, mostly from the hides of cows and pigs. Cellulose casings are made from plant fibers and are often used for larger sausages like bologna and summer sausage. Plastic casings are the least common type of casing and are used for lunch meat and bologna.