Milk is a nutrient-rich liquid food produced by the mammary glands of mammals. It is the primary source of nutrition for young mammals before they are able to digest solid food. A bag, on the other hand, is a common tool in the form of a non-rigid container. The use of bags predates recorded history, with the earliest bags being no more than lengths of animal skin, cotton, or woven plant fibers, folded up at the edges and secured in that shape with strings of the same material. In addition, using a bag to put milk inside a bag of milk inside a bag of milk is a modern problem which requires a modern solution.