Laura Faye Newlon, age 10, of Irvington, Virginia, for her question:

Why does clear ocean water look blue?

It is true that a cupful of seawater looks clear, or almost clear. But scientists tell us that all water has a very faint blue tinge, too pale for our eyes to see. In large quantities of water, such as oceans and large lakes, there is enough to the natural blue to show. However, there are other reasons why the shimmering ocean sometimes is blue. On a clear day, its shiny surface acts like a mirror and reflects the blue of the sky.

Another touch of blue is added by sunshine. Some of the colored rays of light are sifted out as the sunbeams sink down. This leaves a layer of deep blue down below. This shines upward and adds extra blue to the surface. But on cloudy days, the sea looks grey because its shiny surface mirrors the grey sky. And in the evening, the sea even mirrors the flaming colors of the setting sun.