A hydration bladder is a flexible reservoir used to store water or other liquids, commonly found in outdoor gear like backpacks. It typically features a wide opening for easy filling and cleaning, along with a tube that allows users to drink hands-free while on the move. These bladders are popular among hikers, cyclists, and athletes, as they provide a convenient way to stay hydrated during physical activities. Many hydration bladders are made from durable, BPA-free materials to ensure safety and longevity, making them an essential accessory for outdoor enthusiasts.