There are more than 600 species of oaks found growing around the world, and every one of them produces acorns. Acorns are different from other tree nuts because of their interesting two-part shell. The nuts have a smooth shell that is held inside a second cup-like shell called a “cupule.” This “shell and cap” structure is much different from other tree nuts like hickory and walnut, whose inner nut shells are surrounded by outer husks. Acorn cups (or caps) can have very different sizes, shapes, and textures, based upon the species. Some of them are shallow, covering very little of the nut’s shell. Other kinds of caps cover half of the shell, and some almost surround it.