Kwakkel, who teaches at University of Leiden, has spent years examining some of the planet's oldest manuscripts and was amazed to discover the types of things – smiley faces, geometric shapes, random squiggles – medieval scribes and readers would scribble in the margins or back pages of books.
"It's really quite amazing and brings you closer to someone who died a thousand years ago," he told interviewers with NPR's How To Do Everything. "It's quite phenomenal to see that for the first time."
Although some of the doodles Kwakkel has catalogued were made by bored readers, most were created by medieval scribes, testing the ink flow of their pens and quills, prior to copying a book.
"You are actually looking at monks who were in a cell somewhere in a monastery in the middle of nowhere, who grabbed their inkpot to draw a smiley face," Kwakkel said. "It [the doodles] become just as important, I find, as the text that surrounds it."