Martha Ann Hennigan, age 9, of Florence, So. Carolina, for her question:

Why are rain clouds black?

A cloud is made of tiny droplets of moisture and when it gets ready to rain, they get bigger. This is what makes a rain cloud turn dark and still darker. A fluffy white summer cloud is made of tiny droplets of misty moisture. They are so small that they can float in the air with plenty of space between them. There is room for the sunbeams to dance around, bouncing from droplet to droplet. The sunbeams play tricks in the fine misty cloud, tricks that make the cloud look white and fluffy.

All that cloudy moisture is really water. And sooner or later the misty droplets clog together and form larger droplets. As these gobs of water grow bigger, they cast grey shadows. At last they grow too heavy to stay up in the air. They become raindrops, ready to fall. And the raindrops are big enough to shut out the sunbeams and fill the cloud with their dark shadows.