That is a hard question! But it isn't a foolish one, by any means. It was only about fifty years ago that a great English scientist named John Tyndall worked out the puzzle. See if you can understand the answer.

The sun is 92,897,000 miles away from us, but the sun's white rays come to us straight, with no interference, until they strike the earth's atmosphere about forty miles above our heads. This atmosphere absorbs, or swallows up the light rays. If this were all the sun should look to us like a great star, and the sky at midday should appear dark and clear as on a winter night, with all the stars shining. Long ago scientists noticed that on very high snow-covered mountains, and out in mid-ocean where the air is purest, the sky is darker than it is over low land. The air near the earth, that is heavy enough to be breathed, is full of fine earth-dust. These dust particles catch and break up the light rays, just as a glass prism, or a diamond breaks a ray of white light up into rainbow colors. Now if all those rays could get through to us we would have a rainbow sky. But the impurities are of just the right size and number to throw back all the other colored rays, and to reflect the blue rays to us.

So of all the sun's light, we get only the blue rays reflected from the little dust mirrors in our own atmosphere. A blue sky is the very nicest kind of sky for us. If there were no impurities, or dust, in our atmosphere it would be so dark we couldn't see very well. And if the impurities were of a different size or number, we would have a red or a yellow sky. Either one would dazzle our eyes. Now, when our sky is gray it is because the earth dust is coated with vapor or water dust, making clouds. Vapor is not as good a reflector as dust, so we do not get our blue light until after the vapor has condensed and fallen in rain. (See Sun, Tyndall, Spectrum.)