Why is there visible vapor when we exhale during cold weather?

When air is exhaled by the lungs it is saturated with water vapor (100% relative humidity) and at body temperature (about 98.6F). When exhaled into very cold air there is too much vapor in the air and some must condense (like fog) until it is at 100% relative humidity at the colder temperature. The condensed vapor quickly evaporates and disappears into the cold dry surrounding air. The tailpipes of cars may also show condensed vapor on a very cold day because when a gallon of gasoline is burned it produces about a gallon of vaporized water.