The balls in the picture are not roasted potatoes. They are not seeds pods. They are not puffball mushrooms. They are called oak galls. When you find them on the ground, as I did as I walked around Walden Pond, they are hollow and fragile. Each one has a tiny hole in it. Out of each tiny hole, a newborn wasp has crawled.
Oak galls are strange little houses for gall wasp larvae. They work like this: A gall wasp lays her eggs on an oak tree. When she does this, she releases a chemical that causes a reaction in the tree. It forces the tree to grow this weird cocoon-like "apple" around each larva, to protect it. While there's a larva inside it, the apple is dense and spongy. Once the wasp hatches, it drops from the tree and starts to hollow out.
Have you encountered any weirdo science this summer?