A water bed is a type of mattress filled with water, designed to provide support and comfort while sleeping. The water inside can be adjusted for firmness, allowing users to customize their sleeping experience. Water beds often come with a protective cover and may include features like temperature control to enhance comfort. These beds were popular in the 1970s and 1980s, often associated with the hippie culture of that era. While they have declined in popularity, some people still prefer water beds for their unique feel and potential benefits for back pain relief.