I had this mystery waiting for you, despite what Nutmeg thought yesterday!
Know what it is? Give your guess in the comments!
As one of our regular readers noted in the comments today, this is an oak gall, sometimes called an oak apple gall. We had no idea that humans in the middle ages made ink out of them! Humans used to make many things out of plants, and we squirrels feel it would be helpful to the earth if they returned to that habit!
Galls are formed by insects invading plan tissue, either to feed or lay eggs. The plant then begins to grow abnormally in that spot. In the case of the oaks, a wasp has laid an egg in the bud of a leaf. Split the gall open early enough and you should find the larvae of the wasp in the center. It’s interesting that different wasps in both Europe and North America lay eggs in oak trees on both continents and cause oak galls.