Nicholas Lawryk, age 10, of New Brunswick, New Jersey, for his question:

How can dry ice make it rain?

A cloud is made from zillions of mini droplets of moisture, small enough to float aloft in the air. You would think that they can clog together and form sizeable raindrops with no trouble at all. But actually it is not easy to cross the airy spaces between them. And without help, the misty moisture fails to form raindrops large enough to come falling down.

In nature, help is provided by dust, soot, sea salt and other tiny solid fragments. As these tiny seeds drift through the cloud, the misty droplets can collect on them and grow big enough to qualify as raindrops. Dry ice is frozen carbon dioxide and rainmakers chomp it into fine fragments. When these are strewn into the right kind of cloud, misty droplets collect around them just as they do around fragments of dust. And sometimes a dry ice seeding turns a misty cloud into a drenching rain cloud.