An exobiologist studying another planet discovered a group of organisms, which she dubbed "smileys." She carefully drew all the smileys she encountered and numbered them for convenience. The applicable pages from her notebook, which we will be consulting in the next few panels, are reproduced below. You can scroll down to view all fourteen drawings, or you may wish to print this page for your convenience.
The smileys differ from one another in several ways; indeed, no two are identical. Nevertheless, they exhibit only one difference in kind, which obliges us to divide the concept "smiley" into two sub-concepts. Before advancing to the next page, can you identify this one difference? Perhaps you can think of suggestive names for the sub-concepts. 
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