Catherine M. Zaharias, age 9, of Gary, Indiana, for her question:

How do they judge the age of the earth?

Once upon a time, the experts thought that the earth was about 6,000 years old. Then someone remembered the seas. The first seas were filled with fresh water and the salt has been added year by year. This must have taken ages. So it was decided that the earth must be millions of years old.

About the oldest things on earth have to be its hard, hard rocks. Modern scientists learned how to figure when the different rocks were made. They found many that are 100 million years old. They found some that are two billion years old and one that is more than three and a half billion years old. Maybe still older rocks will be found. In any case, they know that the earth must be a great deal older than this old rock. They figure that our planet most likely was born about four and a half billion years ago. In round figures this is 45, with a string of eight zeros.