Happy birthday! You have grown a little more. Everybody has a birthday party every year. But what about trees? Trees don't have birthday parties. They have rings(年轮), How can we know a tree's age? （1）We can count its rings. The rings are inside the trunk (树干) , A ring has a light part and a dark part. The light parts show winter and spring. The dark parts show summer and autumn. Each tree ring means one year. So an old tree has many rings. （2）If you want to see the rings inside a tree, you need to cut it down. But no one wants to do that.
