A small cluster of oak galls on a bur oak sapling. These are made by one of the tiny Cynipid wasps, likely Disholcaspis quercusmamma. The female lays her eggs on the twig thus stimulating the formation of these hard woody galls which form a protective case for the developing larvae. However, the protection is not absolute for other wasps can penetrate these and parasitize the Cynipid larvae. You can see the exit holes on the bottom, but whether cynipid wasps appeared or a parasitic wasp, I don`t know.