Michael Main, age 9, of Grimes, Iowa, for his question:

How is sea level determined?

Sea level would be very easy to determine if the global ocean would stay perfectly calm and still. But, of course, it does not. The waves are tossed by stormy winds. Twice a day, high tides and low tides chase each other around the world. So scientists figure that the average sea level is halfway between the highest high tides and the lowest low tides. To get the answer, they have to take many measurements of high water and low water along the shores around the world. These figures are computed to determine the midway point of mean, or average, sea level.