Originally Posted by Evil Q
Wouldn't a hot water bottle get, well, f'ing cold overnight?
No. The rubber keeps the heat in, and if you have a cover that keeps it warmer for even longer. I'd guess that a hotty botty could stay hot for about an hour, and takes at least two hours to cool down, if it's kept under the covers. Even a cool hot water bottle is never "f'ing cold" -- when hot water loses its heat it turns to "room temperature," which, if it's under your blankets, is still pretty warm.
That said, a hot water bottle that is no longer hot is a burden, and should be cast out of the bed (usually with a satisfying "plop"). There was a joke in the TV series Maid Marrion and her Merry Men
in which King Richard threw a pig out of his bed saying "my hot water bottle is cold" (they didn't have rubber botties in the middle ages; only pigs).