Geoffrey Lea, age 12, of Kendall Park, New Jersey, for her question:

How does the dew form?

Some people suspect that it falls from above, perhaps like gentle rain. Others suspect that it oozes out from the ground and the greenery. Neither of these theories is correct. The morning dew forms when moisture in the air condenses on cool solid surfaces. The same thing happens when a glass of ice water sits in a roomful of warm, moist air. The outside of the glass gathers a mist of dewy sweat.

These things are caused by rules that govern the water vapor content of the air. Warm light air is allowed to contain more gaseous water vapor than cool, dense air. As air warms up, it gets thirsty and evaporates more moisture into gaseous vapor. Soon it absorbs all it can hold and reaches its saturation point. If this air cools off, it can hold less vapor and some must be converted to liquid droplets. As a rule, the temperature begins to fall after sundown and reaches its lowest poing just before dawn. Objects near the ground become chilled and they chill the air that touches them. If the air is loaded with vapor, this condenses in drops of pearly dew