How to keep warm

Usual Friday night sleepover for E and J, but E was feeling a bit unwell and asked if we had a hot water bottle. We don't have any hot water bottles, but I remembered we had this antique.  I filled it with hot water, but I guessed it wouldn't be watertight so when the bottle had heated up I poured out the water and wrapped a thin towel around it so that she would not burn herself as the bottle was quite hot.  Problem solved.  I don't know how many times I've thought of throwing the bottle out.