Mark Gary Rubin, age 10, of Staten Island, New York, for his question:

Is it true that the ocean once was fresh water?

The scientists say that this was so    and they have a lot of evidence to prove it. The first oceans were formed billions of years ago, when the crust of the infant earth began to cool. Torrents of fresh rain water deluged down from a skyful of dense, dark clouds. It rushed down the bare slopes, settled in the hollows and filled up the deep ocean basins. Much later, the skies cleared. The sun glistened down on the wet ground and the sparkling oceans of fresh rain water. But the changeable weather soon got busy adding salts and all sorts of other dissolved chemicals to the seas. And this is still going on:

Running water dissolves soft minerals as it gushes over the ground. Streams meet to form rivers and rivers flow to the sea. All this so called fresh water carries along invisible traces of dissolved salty chemicals    and dumps them into the ocean. The sun evaporates moisture from the seas, but the dissolved chemicals stay behind. Meantime the rivers are dumping more salty minerals into the ocean, and there they stay. The first seas were filled with fresh, deluging rains. But every year, the running rivers added more and more dissolved salty chemicals to their waters.