Danny Galli, age 11, of Portland, Maine, for his question:

How do they make it rain?

Weather experts are not very good at making rain but they hope to improve with practice. The methods they use try to copy the way in which nature turns cloud moisture into drops large enough to fall as rain. A cloud contains tons and tons of moisture in the form of minuscule droplets. Thousands of these droplets must meet and congeal to form a single raindrop. This is not easy because they are widely separated. As a rule, small solid fragments are needed to act as nuclei around which the droplets can cling.

Meteorologists add supplies of suitable nuclei to a likely cloud    and sometimes make it rain. This so called "cloud seeding" may be done by spreading fragments of dry ice through a moist cloud. It may be done from an airplane. Sometimes small crystals of silver iodide are sprayed into a likely cloud. This may be done by powerful generators that shoot smokey silver iodide up from the ground. Successful rain making, however, depends on very many known and unknown weather conditions.