Despite the name, pencils have never been made with lead. They’re made with graphite, a form of carbon. Just where does graphite come from, and why did we decided to write with it? NPR took a look into the history of pencil lead, and its enemy, the eraser, shooting an eye-candy video inside a factory owned by the General Pencil Company.

Henry David Thoreau, that master of American literature, had perhaps an even more profound effect on writing instruments than he did on writing. Thoreau, whose father ran a pencil factory, decided to start mixing graphite with clay, discovering that different amounts of clay produced lighter or darker shades, and that the mixture resulted in stronger, less smudgy marks. That variation in clay gave us the numbering system that you’ll recognize from being repeatedly warned not to use anything but a No. 2 pencil on your standardized tests.

And what is a pencil without an eraser? The humble eraser has played a bigger role in the modern world than you might think. Before commercial erasers, people used crumbs of stale bread to rub their writing mistakes away. In 1770, Joseph Priestley—who, among other accomplishments, invented soda water after he discovered oxygen—was the first person to realize that a certain South American tree produced a gum that could erase pencil marks more effectively than balls of bread. Because the process involved some rubbing, he named it “rubber,” although the material would later be used in plenty of applications that don’t involve rubbing at all.

Watch the video below for an inside look at a pencil factory and the machinery that churns out the writing instruments we use every day—or did, before computers took over the world.

[h/t NPR]

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