It is a combination book holder AND bookmark. You slide it on your thumb and it allows you to hold open a small book with one hand. When you're done, the flat part slides into the book as a bookmark. (I have a bunch of these.)

I always hoped that Watterson would make an old-fashioned Christmas special or something (not a sat morning cartoon). C&H probably had a bigger influnce on my childhood than any other form of contemporary culture. It's too bad he has no interest in ever returning to the characters again.