Milk in a bag comes in a few forms. One is the half gallon type that you would by for home use. There is a reusable pitcher that the bag goes in. This saves material in the manufacturing process, making it cheaper. Another type is the 2 to 5 gallon bag, this is used in many restaurants and they are put into large dispenser machines that use a weighted lever to close off the "hose" that comes out of the bag.