I thought we all knew that?
I always thought the idea was to have two bottles on the go at the same time: one frozen in front of the fan and another one in the freezer freezing to replace the other one when it thaws out.
(Plastic bottles only, please, if you're minded to give it a bash).
Something I heard the other day: have two fans going simultaneously, one in front of you and one behind you. Apparently the circulation of air around your torso cools you down.*
* Might be cobblers. Haven't tried it for myself and have no idea if there's anything to it.