Almost everyone with young children has been through the "How," "Why," and "Where" period.

How can a fish breathe water? It doesn't really breathe water, Victoria. It breathes air the same as we do, and there is a little bit of air in most water. The fish's gills extract the air it needs.

Why are plants green? They are green, Victoria, because the plants contain a green material called chlorophyll. The chlorophyll makes use of the energy in sunlight to produce the materials that the plants need in order to grow.

Where does the sun go at night? It shines on a different part of the Earth. The Earth is like a round ball, so big that to us it looks flat, and it spins around and around, one complete turn in one day. It's daytime on the side that faces the sun, and night on the side that faces away from the sun.

And then, just when you feel you are doing pretty well, the five- or six-year-old asks, Why is the sky blue?

This question can be irritating in two different ways. First, you may not know the answer. Second, even if you do know the answer it's not easy to reduce it to a form that a five-year-old can understand.

Here is the answer that I have learned to offer over many years of trying. At the end, the child is either satisfied or has totally lost interest in the subject.

Sunlight looks kind of white, but actually it's made up of many different colors. You can see those colors in an oil slick sitting on top of a puddle of water, or by letting sunlight fall onto a triangular block of glass called a prism. Or, most spectacular of all, when the sun is shining and it is raining at the same time, you may see a rainbow. The rainbow colors change from red on the upper side of the arch, to deep blue and violet on the lower side.

What causes a rainbow? This is another awkward question that we will dispose of on the way to the blue sky. A rainbow happens because rays of different-colored light are bent different amounts when they go through the little spherical drops of water that make up rain. They bend when they enter the water drop, are reflected inside the drop from the drop's outer surface, and then bend again when they emerge. This means that the light rays come out in certain preferred directions. These directions form a cone relative to the observer on the ground. That cone is a little wider for red light than for blue light, with other colors occurring in between, so you get, as you would expect, all the colors of the rainbow within the rainbow.

If you want to know where in the sky to look for a rainbow, face the sun and imagine a straight line connecting you and it. If you continue that line behind you, that direction forms the centerline of the rainbow's cone. Since the sun is above the horizon and you are down on the ground, the center of the rainbow's cone lies in a direction that is below ground level relative to you. That's why you normally see only an arc, a small part of a full circle.

Even when it's not raining, though, the molecules and tiny particles in ordinary air bend light rays. The violet and blue rays are bent the most, the red ones the least, the same as in the case of a rainbow but more so. However, we no longer have near-perfect spheres, as with water drops, so there are not the same preferred directions for bent light rays. Blue light is scattered from sunlight, then is scattered again by other molecules so that it comes to us from all over the sky. Red light is not scattered nearly as much, so the light that comes to our eyes from the sky has lots of blue and a negligible amount of red in it. And that's why the sky is blue.

If the child is still listening, you end your explanation and take a bow.

However, I can't leave the subject of rainbows and the bending of light rays without mentioning one other thing. I call it the moonbow, though there may be a more official name.

Sometimes it rains on nights when there is a moon. The full moon is far less bright than the sun (about 400,000 times less bright, which gives some idea of the astonishing adaptive power of the human eye). However, rays of light from the moon will be bent by water droplets, exactly as the sun's rays are bent. The result ought to be a moonbow, a much fainter version of the rainbow.

Why have I never seen such a thing, although I have looked for it? I can think of two possible reasons. The first is that it is simply too faint to be seen at all by human eyes, although presumably a camera with sensitive film could catch it. The second possibility is that I have indeed seen a faint bow of light in the sky, but paid no attention to it because it was white, rather than colored.

And why would it look white? We have to remember that the retina of our eyes contains two different kinds of light-sensitive objects, the rods and the cones. The cones work best in bright light, and they are responsible for all color vision. The rods work at low light levels - nighttime conditions - but see only in black and white.

It may be that the moonbow is there, full of color, but the low level of light makes human eyes unable to see it. Which leads me to my final question, one that I hope no child ever asks me because I can't answer it: Do cats and owls and other nocturnal creatures regularly see a magical colored moonbow on rainy moonlit nights? If they do, I envy them.

"Borderlands of Science" is syndicated by: