Gayle Siegel, age 11, of St. Paul, Minn., for her question:

What makes a pitcher of water sweat?

Sometimes the outside of a pitcher of ice water gathers a coating of beady sweat. This is caused by a trick of air and moisture. This very same trick operates outdoors on a grand scale. It causes clouds to form in the sky and dew to gather on the grass.

When sweaty moisture gathers outside a pitcher, we might take it for granted that the water it contains is seeping through the sides. Then we recall that the pitcher I has sides of glass or same other material too solid to let through even a drop of moisture. We look for cracks and there are none. We check further and find that no one sprayed the beady droplets on the outside of the pitcher. At this point we know we have a mystery to cope with, and the time has come to ask a question.

The answer explains a trick performed by the filmy air, its temperature and its content of water vapor. There is a certain amount of this gaseous water in the air, even above the driest deserts. This vapor is a gas made of separate water molecules. As a gas, it mingles with the mixture of other gases in the air.

But there are rules as to how much water vapor the air can hold. The rules depend upon the temperature of the air, and like all rules of nature they must be obeyed. Warm air can hold more vapor than cooler air. If warm air having its full quota of vapor is cooled, it is left with a surplus of vapor. This surplus vapor must be changed back into liquid water.

There is vapor mingled with the air in a warm room. When the water in a pitcher is colder than the rowan, it chills the warmer air touching its sides. This layer of cooled air now has a small quota of surplus vapor. Some of its gaseous water must be changed into droplets of liquid. And these foggy droplets of liquid water gather on the cold outside surface of the pitcher. Covering it with a cloud of beady sweat. A pitcher of very hot water does not sweat, and a pitcher of water does not sweat when placed outdoors on frosty winter day.

The weather air outdoors gathers masses of evaporated water vapor and its temperature rises and falls. When vapor filled air rises and cools, its surplus water gas becomes the misty droplets of liquid that form the clouds. When the morning air touches the chilly ground, its surplus vapor becomes pearly drops of liquid dew.

There is a strict ratio between the temperature of the air and its content of water gas. The weatherman calls this relative humidity. When air has all the vapor it may hold, it has reached saturation point. A cubic meter of air at 86 degrees fahrenheit is saturated when it contains 30.4 grams of moisture. The same air at freezing point is saturated when it contains only .85 grams of gaseous water.