I went hiking, with Peter and Jim, north of Cave Creek, Arizona last Sunday and one of the wonders we came across was an oak tree with oak galls on it. The ones pictured above are about a half inch in diameter. They were mostly empty and the shells weigh less than a gram. They appeared to be growing on leaves. Sometimes called oak apples they are formed by wasps and are related to their reproduction cycle. Until about 100 years ago they were a major ingredient of ink. There is lots more about oak galls on Wikipedia. It reminded me that I need to use my Backyard Scuba techniques more often. Thank you Peter B.