Explain that the formation is called dew. When moist air contacts a cooler surface, such as the side of the can or the leaves of a plant, moisture from the air condenses on the surface. Half-fill the second can with water only. Use two thermometers to record the temperature of each can. Then stir the contents of both cans rapidly.
Students can observe the outside of the cans.
Explain that the formation they observe on the first can is called frost. Compare the two temperature readings, recognizing that when the surface of an object is below freezing, the water, vapor condenses on the object as frost. Make some smog by dropping a burning match into a clear half-gallon bottle with your mouth pressed tightly on the mouth of the bottle.