A tree grows well when there is lots of sunshine and rain. And little sunshine or rain will make the tree grow slowly. Trees can tell us the history（历史）of the weather. For example, to find out the weather 10 years ago, count the rings of a tree from the outside to the inside. If the 10th ring is far from the 11th one, we can know that it was sunny and rainy most of that year. If it is near the 11 the one, there was little sunshine or rain that year.
Tree rings are important not only for studying（研究）the weather but also for studying history. Many years ago, there lived a lot of people at a place in New Mexico（美国新墨西哥州）.
But now there are only sands there, no trees and no people. What happened? A scientist studied the rings of dead trees there. He found that the people had to leave because they had cut down all the trees to make fires and buildings. Because all the trees had gone, the people then had to move.
