Cynthia Devine, age 11, of St. Louis, Missouri, for her question:

What makes a star twinkle?

The starry sky is a wondrous, breathtaking sight to behold. But sooner or later you recover your curiosity and begin to notice a few questionable details. For example, the bright stars appear to shoot off shafts of light. They twinkle with winks and blinks. This is puzzling, because the stars are millions and millions of miles away and from such distances we could not possibly see such details. Actually, though, the light comes from the distant stars, and the twinkles are added by the earth's atmosphere.

In our enormous atmosphere, there are layers of dense and light air, cool and warm air, damp and dry air. These airy assortments are in con¬stant turmoil. And starlight must pass thorough hundreds of miles of this airy mishmash to reach us. On the way, its rays are bent this way and that way, by masses of different air layers. The light rays zigzag and the stars seem to wink and blink with merry twinkles.