A pine tree ( 松树 ) in California was regarded as ( 被认为 ) the oldest tree in the world. People thought it was nearly 5,000 years old, but they could not be sure. Then someone cut the tree down with a chain saw ( 链锯 ). Killing the oldest tree in the world was a silly thing to do. But when the tree was cut down ( 砍下 ), everyone knew its age. It was 4,900 years old. How can you tell the age of a tree? If you ever see a tree that has just been cut down, take a look at the cut part. You will see rings ( 年轮 ). There are little rings in the middle. Around each ring near the bark ( 树皮 ) is the biggest one of all. Count ( 数 ) these rings. A sixty-year-old tree has sixty rings. How many rings does a 4,900-yare-old tree have?