Karen Ball, age 10, of Asheville, North Carolina, for her question.

What happens to the blue sky at night?

A very popular 10 year old question is: Why is the sky blue? Many young pen pals have asked for the answer. However, for each question, Andy can select only one pen pal    and he can answer questions only in the column. So, please do not feel sad or mad if the person selected was not you. Chances are, a zillion good questions pop into your head. You can send in as many as you wish and, who knows; one of them might be selected. In the meantime, today's answer also explains your old favorite about why the sky is blue    at least in the daytime.

The sky, of course, is really our view of outer space. And this endless ocean of space reaches out from our round world in every direction. If you were out there, way beyond the earth, the ocean of space would be black, dotted with dazzling stars. Black is the true color of space    so midnight black is the true color of the sky.

While you were out there in space, you could look back and see our gorgeous round planet. You might be able to trace the geography of some of its lands and seas. But most likely a lot of the details would be blurred or even hidden by clouds. These clouds have a secret clue that helps us to understand why the blue sky loses its color after the sun sets. Clouds are part of the atmosphere    and this enormous blanket of air is wrapped around the entire globe.

Now, let's return our daydreams to the solid ground. When we look up, we see the vast ocean of space through a thick layer of air. It veils and blurs our view of the sky. Even at night it steals some of the blackness from the true color of outer space. But in the daytime, our airy atmosphere plays more colorful games with the sunbeams. As they streak down from the sun, the air above us bends off their blue rays and scatters them over the sky.

This is why the daytime sky looks blue. At night, of course, the sun is not up there to shed its sunbeams. With no sunbeams to play with, the air cannot scatter their blue rays all over the sky. Then we see almost the true color of the spacious sky.

All this seems strange because the sunshine that lights the day seems to have no color at all. Actually, it is a skein of rainbow colors, blended together so that they lose themselves and become invisible. Each color is a different wave length of pulsing energy. The shortest wave lengths are blue rays, the longest ones are red. Air,  of course, is made of separate gas molecules. They are much too small for our eyes to see, but just big enough to bend some of the wavelengths of light. In the day time, they bend and scatter the blue rays over the sky. But the blue disappears when the sun takes the sunbeams off to bed.

These airy tricks are very mystifying. You can see clear through the air, at least for a mile or so. It's hard to believe that its bitsy particles can blur the view. But they can. A few miles of air between you and a faraway mountain casts a filmy haze in the distance. The air above our heads reaches up many hundreds of miles.