BTDT.

Camping in the open on the coldest night of the year with a substandard sleeping bag. Sat by the fire all night drinking tea and tucking hot rocks down by my feet. I put them in a canvas bag, but still managed to scorch a few holes in it.

It works, but its hard to tell the difference between hot enought and too hot, especially when numb with cold. I agree that a hot water bottle is probably safer.