Originally Posted by grok
when it cools wouldn't it then draw out heat from your body?
You're on the right track. Here is the issue when it cools off:
When it cools to or below your body temperature, your body is now keeping it warm. In return, the bottle of water is effectively increasing your bodies surface area by it's area minus the area it comes in contact with you (pretty much the surface area of the bottle. This increased surface area is able to give off more heat than your body is alone... But in all honesty this is pretty negligible.
With that said, once the bottle doesn't feel warm any more, ditch it or swap it out for a hot one =)