I wasn't sure freezing it would work at keeping the moldies from growing, but it DOES. I empty all the water, throw the whole thing in the freezer (outside pack and all) and it's not gotten moldy once since doing that (and the outside pack doesn't smell as bad because I guess freezing it kills the sweaty germs, too). I only ever use water in it, but I'd still get mold in there. Not any more. Thanks ladies for the tip!!!