Anyone who has ever taken an SAT has had the sacredness of the No. 2 pencil drilled into them. But what makes a No. 2 pencil what it is? What separates it from a No. 1 or No. 4? And what makes a pencil—well—a pencil, anyway? This video, from NPR's excellently named Skunk Bear science channel, offers all these answers and more, in a great, four-minute explainer that harkens back to the best of Mister Rogers's Neighborhood's "Picture Picture" segments.
What makes a pencil a pencil is graphite, of course, a crystalline semi-metal that was once known by the fascinating name "plumbago." Like oil, coal, and diamond, graphite is an allotrope of carbon; in other words, it's formed when a carbon deposit undergoes a certain amount of pressure over time. What makes graphite unique is that the various sheets that make up its crystal-lattice atomic structure have a tendency to shear off under friction.
That's a quality that 16th-century farmers took advantage of, when a lightning storm uncovered a massive deposit of graphite in Cumbria, England. As Skunk Bear's Adam Cole explains, the farmers sound discovered that chunks of graphite allowed them to mark their sheep, separating one flock from another on the rainy English hills. They mistakenly started calling the material lead. The name stuck, the natural philosophers of later eras—most notably Henry David Thoreau—soon realized graphite was its own thing entirely. Thoreau discovered that you could make cheaper, more dependable pencils by mixing inferior-grade graphite with clay. You could also control the lightness and darkness: "Harder pencils" contained a greater clay to graphite composition, leaving lighter marks; softer pencils left darker marks because the ratio was reversed.
Cole explains that it was Henry David Thoreau's father, John Thoreau—owner of a pencil factory—who began classifying the softness of pencils by their numbers. No. 1 pencils were the darkest; No. 2 were the second darkest, and so on. Today, that scale includes 21 different grades, but No. 2 pencils are still the ultimate "tweener" pencil: the perfect middle ground between hard, light pencils and dark, soft pencils—and the perfect pencil for standardized test reading machines, too.
Check out Skunk Bear's full, fascinating history right here.