h2g2 The Hitchhiker's Guide to the Galaxy: Earth Edition

Another good use for towels is the prevention of dehydration. If one finds themsef without shelter or transportation, traveling on foot in hot weather can be exceptionally drying. If water or other similar beverage is either limited or not on hand, it is wise to conserve the water present in one's body. By lightly covering one's mouth and nose, one can trap a good deal of the moisture lost in respiration. As one exhales, this moisture is held by the towel and when one inhales, some of that moisture is returned to the lungs, thereby limiting the loss of water. Of course, this will often result in a musky odor and one should also make certain that the towel doesn't too heavily cover the mouth and nose and pose as a potential suffocation risk.

Also, in similar weather, to keep cool, a slightly moistened towel can be draped around the sides and back of the head and held in place using some variety of belt or cap. The latter option also affords the hitchhiker the added benefit of sheilding his or her eyes from sunlight as well as providing shade for their face.