Don Johnston, age 12, of Seatle, Washington, for his question:

How long has there been water on earth?

The latest estimates suggest that the old earth has had between three and four billion birthdays. Most experts agree that it began as a seething ball of gasses, too hot and too hazy for water to exist. No rain fell we are told, until the solid crust of the earth began to harden. And the first rains, fell on rocks so that the water turned immediately to steam.

At last the surface did cool enough to hold the falling rain. Exactly when this happened no one knows. Certainly the world was already several million years old, though still in her babyhood.

Those early rains deluge down day and night for untold centuries. They gushed over the land and poured into deep hollows which became the ocean basins. The seas were born at that time and have not dried up sineeo so the world has had a plentiful supply of water almost from the very beginning ‑ which was over three billion years ago.