This soft, flexible "bottle" can be rolled up and folded when not in use. The Flex Water Pack is made with an extremely strong material that is resistant to breaking. In addition, an antimicrobial compound has been added to the material that releases silver ions in the presence of water. This helps prevent the growth of bacteria when the Flex Water Pack is at the bottom of your backpack when out on the trail or when hanging up in your kitchen drying. Please note this is not a substitute for regular cleaning. The Flex Water Packs can be frozen. *Do not add hot water.