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Croation inventor Eduard Penkala patented the rubber hot water bottle–which he dubbed the “Termofor”–in 1903. However, it did not come out of nowhere. In fact, the history of the hot water bottle goes back thousands of years, albeit in different guises. https://www.lowtechmagazine.com/2022/01/the-revenge-of-the-hot-water-bottle.html