That is a hard question! But isn’t a foolish one, by any means. It was only about 70 years that a great English scientist named John Tyndall worked out the puzzle. Let me know in the comments section at the end if you can understand the following 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 40 miles above our heads. This atmosphere absorbs, or swallows up the light rays. If this were all, the sun would 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 could 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 only get 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 could be so dark that we couldn’t see very well. And if the impurities were of a different size or number, we could have a red or yellow sky. Either one would dazzle our eyes. Now, when our sky is grey, it is because the earth dust is coated with vapor or water dust, making clouds. Vapor is not a good reflector as dust, so we do not get our blue light until after the vapor has condensed and fallen in rain.