It's this way: You remember how you can "see your breath," as we say, on a cold morning? Well, that's because the moisture in your breath is condensed by the cold. Now as the waters of the earth--the seas, lakes, rivers, ponds, and so on--are warmed by the sun, the air above them is filled with moisture, for the heating of the air causes it to expand and draw in moisture from the water like a sponge. Expansion makes it lighter also, and it rises. Rising, it turns cooler, and the moisture condenses and comes down as rain. Mountains usually have clouds around them because moist air striking the mountainside is driven up the slope, cooling as it rises. So rain and snow fall often in mountain regions, and that's why so many rivers rise in mountains. The moist air is also condensed when it meets other and cooler air currents. But right here is where the work of the dust comes in. For to make rain you've got to have clouds, and clouds are due to this moisture collecting around the little particles of dust of which the air is full. When these little motes of matter become cooler than the air that touches them the moisture in the air condenses into a film of water around them. Fairy worlds with fairy oceans floating in the sky! Each of these baby worlds is falling toward the big world below. But very slowly; only a few feet a day, so that even if nothing happened it might be months--yes, years--before it would come to the ground, even in still air. But when air is very thick with moisture the water films on these dust particles grow rapidly, and thus increasing in weight, they fall faster and faster, and finally strike the earth as raindrops. But here's another thing that helps. On the way down two or more raindrops, falling in with each other, will go into partnership--melt into one--and then they hurry down so much the faster. That's why the sky grows darker and darker just before a rain, and why the lower part of a rain-cloud is the darkest: the little raindrops are forming into bigger raindrops as they fall. THE LITTLE ARTISTS THAT SHAPE THE CLOUDS But the shapes of clouds are supposed to be due to another thing, the mysterious force we call electricity, and that other mysterious force we call gravity. Just as the worlds attract each other by gravity so these raindrops--or dust grains growing into raindrops--are drawn toward one another. Here's where Electricity steps in. These rain particles are full of electricity and when two of these electrified particles meet in the air--unless they strike one another in falling, in which case, as I said a moment ago, they blend into one--they get very close together and yet keep dancing around one another without touching! It is this dancing about that makes all those strange and beautiful and ever-changing forms in the vast picture-gallery of the sky. Of course the wind currents help to change these shapes, but I'm talking about the original designs. II. THE RAINDROPS AND THE RIVER MILLS