How do subtrees demonstrate the recursive nature of trees?

"Finally, much of what gives trees their expressive power is their recursive nature. This means that a tree is made up of other (smaller) trees. Consider our example. It is a tree with a root of A. But the two children of A are each trees in their own right! F itself is a tree with only one node. B and its descendants make another tree with four nodes. We consider these two trees to be subtrees of the original tree. The notion of “root” shifts somewhat as we consider subtrees — A is the root of the original tree, but B is the root of the second subtree. When we consider B’s children, we see that there is yet another subtree, which is rooted at C. And so on. It’s easy to see that any subtree fulfills all the properties of trees, and so everything we’ve said above applies also to it."