Ruth, Pahiatua.

I often use Lemonade/Coke bottles as hot-water bottles. They are made to stay sealed under high pressure so are safe as long as the lid is screwed on tightly and it is easy to regulate the heat you put into them. My children often had 2 or 3 in their beds at a time and we would only fill them enough to be able to squash them a bit to get the air out and so they stayed in place a bit easier. They make cheap and easy foot-warmers for in the car or at the desk too.