What are those small, brown, round balls that are falling on my driveway from an overhanging pin oak tree?
Those are most likely fuzzy oak leaf galls. Oaks are often attacked by a group of small insects called gallmakers, usually wasps or flies. These insects cause deformities in leaves, twigs, flowers, buds, and acorns which are called galls. Each gall contains eggs or larvae which can overwinter in a nutritive, protective environment to assure survival of the species.