We have become so accustomed to certain creations of nature that it seems idle to inquire into their true nature. This is also the case with oak galls (also known as oak apple, oak gall, leaf gall, or oak apple, Fig. 1). Who has not seen them at some point, and who has not played with them as a child? But how many of us actually know what oak galls are? They are an excellent example of the complex interrelationships between plants and animals.