Oak Apple (made by Gall Wasp)

My daughter found this ball-like object next to the trail. We had no idea what it was! We shook it but it did not rattle. It's very light weight, seems to be filled with air and maybe some sort of dry loofah-like structure inside. The oustide was somewhat strong, and smooth like shelac, but seemed very thin - like a ping-pong ball. I could not tell if it was wood or leave or cocoon silk. We did not break it open as we did not know what it was. A week or so later, though we made this spotting, still attached to the tree and the mystery was solved -- http://www.projectnoah.org/spottings/25605193 . It's a dried up oak apple gall that has blown off the tree or fallen with the rest of the leaves.