Its an old trick. I heard about it at least 20 years ago, but didn't try it until last weekend. WOW,....DO IT!!! Nalgene's 1 liter bottles have never leaked on me in the years I've used them and didn't Saturday night. It wasn't that cold outside @ 40 degrees but I wanted to try and be extra toasty. Extra toasty happened. I didnt even bring the water to a full boil like people recommend. the water in the bottle was about 160 degrees. I stuffed in a sock a tossed it in the footbox of my bag. The warmth floating up the bag was much greater than I thought it would be. It warmed my sleeping bag all the way around up to my knee, and halfway up the backside of my thighs with the way I was positioned. 8 hours later the Nalgene was about 90 degrees. No, I don't need a warmer sleeping bag,...ever. It does have me thinking about 16 ounce Nalgenes though.