More about the fascinating history of pencils here: http://www.npr.org/sections/ed/2016/10/11/492999969/origin-of-pencil-lead
Credits: Produced by Ryan Kellman and Adam Cole Senior Editor: Alison Richards Supervising Editor: Anne Gudenkauf Production assistance: Bronson Arcuri, Elissa Nadworny
EXCERPT: "In 1779, scientists showed that pencil “lead” wasn’t lead at all. It was made entirely of carbon. A few years later, another pure-carbon mineral was revealed: diamond. The two substances couldn’t be more different. Graphite is dark and brittle. Diamond is clear and incredibly strong. How could they have the same chemical makeup?
In the 1920s, the answer was revealed. In diamond, carbon atoms are stacked in a pyramid, forming tight, strong bonds. In graphite, carbon is arranged in sheets. Within these sheets, atoms form a sturdy, hexagonal lattice. But the bonds between those sheets are weak — they slide apart with ease. When you drag graphite across paper, those sheets slough off.
The layer of graphite left on the paper is incredibly thin — a thousand times thinner than a human hair. That means, according to mathematician John Barrow, a single pencil could draw a line over 700 miles long." -NPR’s Skunk Bear