A hydration bladder is a flexible water reservoir designed for outdoor activities like hiking, biking, and running. It typically consists of a durable plastic or rubber pouch that can hold several liters of water. Users fill the bladder and then drink through a tube, allowing for hands-free hydration while on the move. These bladders are often integrated into backpacks, making them convenient for carrying. They are popular among athletes and adventurers because they help maintain hydration without the need to stop and pull out a water bottle. Proper cleaning and maintenance are essential for longevity and hygiene.