A bed is a piece of furniture which is used as a place to sleep or relax. Most modern beds consist of a soft, cushioned mattress on a bed frame, the mattress resting either on a solid base, often wood slats, or a sprung base. Many beds include a box spring inner-sprung base, which is a large mattress-sized box containing wood and springs that provide additional support and suspension for the mattress. Beds are available in many sizes, ranging from infant-sized bassinets and cribs, to small beds for a single person or adult, to large queen and king-size beds designed for two people. While most beds are single mattresses on a fixed frame, there are other varieties, such as the murphy bed, which folds into a wall, the sofa bed, which folds out of a sofa, and the bunk bed, which provides two mattresses on two tiers. Temporary beds include the inflatable air mattress and the folding camp cot. Some beds contain neither a padded mattress nor a bed frame, such as the hammock, which is considered one of the most comfortable places to rest while swaying side to side.