Canteens are containers used to store and transport liquids, usually water. They are often made of metal or plastic and have a lid with a spout for pouring. Flasks are similar to canteens, but they are typically smaller and made of glass or metal. They are used to store and transport small amounts of liquid, such as alcohol or other beverages. Both canteens and flasks are commonly used for camping, hiking, and other outdoor activities.