A hot water bottle is a container filled with hot water and sealed with a stopper, used to provide warmth, typically whilst in bed, but also for the application of heat to a specific part of the body.
Containers for warmth in bed were in use as early as the 16th century. The earliest versions contained hot coals from the dying embers of the fire, and these bed warmers were used to warm the bed before getting into it. Containers using hot water were soon also used, with the advantage that they could remain in the bed with the sleeper. Prior to the invention of rubber that could withstand sufficient heat, these early hot water bottles were made of a variety of materials, such as zinc, copper, glass, earthenware or wood. To prevent burning, the metal hot water flasks were wrapped in a soft cloth bag.
Modern day conventional hot water bottles are manufactured in rubber or similar material, to a design patented by the Croatian inventor Eduard Penkala. They are now commonly covered in fabric, often with a novelty design.
By the late 20th century, the use of hot water bottles had markedly declined. Not only were homes better heated, but newer items such as electric blankets were competing with hot water bottles as a source of night-time heat. However, there are now some products available that can still be described as a hot water bottle (because they still contain water, soaked onto a nonwoven material pad), which can be heated in a microwave oven, with consequent cost and safety benefits.
While generally used for keeping warm, conventional hot water bottles can be used to some effect for the local application of heat as a medical treatment, for example for pain relief; but here also, newer items such as purpose-designed heating pads are now often used.