What happens to the roots of baby teeth when they fall out?

The root of a baby tooth slowly dissolves as a permanent tooth grows in underneath. Once most of the root has been dissolved, all that remains is the crown of the tooth, and a small part of the root. The body then reabsorbs this remaining root fragment, after which the baby tooth becomes loose. Eventually, the baby tooth comes completely out of the mouth as a result of the new tooth pushing against the root from beneath.