I use the MaxChill bottles, and they work. They function like the desert water bags, where water wicks through the canvas and cools by evaporation. The MaxChill bottles have a foam cover on them and you apply water from the outside and the slow evaporation cools them. They have to be rewetted every 45 minutes or so. I carry four MaxChill bottles with Gatorade, and a bladder of water. I drink from the bladder and use it to wet the bottles. Gatorade is soooo much better cool.