I always have one with me in my Maxpedition Jumbo. Last weekend I left bag & bottle in the car overnight (by mistake) along with a bottle of a name brand bottled water in the cup holder. We have had numerous night of below freezing weather(usually around 15-18 degrees).To my suprise the water in the Nalgene bottle,while cold,did not freeze and as expected the bottled water did. This weekend I had two Nalgene bottles-and left one unattended on the back seat.This morning my girlfriend thought the water would be frozen when I opened the door (she had no idea it was there-I was leaving and putting my pack in the back seat) and was suprised when it wasn't. From my perspective it's good to know that even at 18 degrees the water is still drinkable (although I do know that the car temp inside is higher than outside-I figured it was probably only about a ten degree increase than outside. Does anyone know at what temp (outside of a car) water in a Nalgene bottle will freeze?