Rebecca Korroch, age 9, of Lansing,Mich. for her question:

HOW OLD IS THE PLANET EARTH?

Our beautiful earth is not about to tell her age. But our scientists are very clever about solving secrets of this sort. For one thing, they look for clues buried deep in the ground. They can tell, more or less, how long the mountains have been growing and how long it took to add all that salt to the seas. They find fossil bones and use a clever way to tell how long they have been buried.

Naturally the earth was born long before the mountains, the seas and everything else. It must be older than the oldest rocks‑‑and some of these are 2 or 3 billion years old. All the evidence helps, but it does not tell exactly when the world was born. But so far as we know, our lovely old planet has had about 4 1/2 billion birthdays.