Why does a pencil leave a mark on paper? The answer is in the way graphite is built. Its sheets of carbon are held together only weakly, so when you drag a pencil across a page, the sheets slide apart and flakes rub off onto the paper. Diamond is made of the same atoms, arranged in a different way, and it is one of the hardest things known. The arrangement is everything, and graphite's arrangement is slippery.

The black part of a pencil was never lead. In the 1500s, graphite was found near Borrowdale in England's Lake District, and it became the first big graphite mine. People thought graphite was a form of lead, so they called it black lead. That old mistake is the reason we still say pencil lead.

The first pencils were not what you hold today. Early graphite sticks were wrapped in string or sheepskin to make them stronger and easier to hold. Later, people set the sticks inside wood. The word pencil itself comes from a Latin word for a small brush, and in the 1700s the word still usually meant a small paintbrush, not a writing tool.

In the 1790s, France was cut off from English graphite, most of which came from Borrowdale. Nicolas-Jacques Conté found a way to use poorer graphite by mixing it with clay. He won a patent for his clay-and-graphite method in 1795. One book calls him the inventor of the modern pencil.

The clay is the key to the whole pencil. The more clay in the core, the harder the pencil, and the more graphite, the softer and darker its line. In the United States, hardness numbers run from No. 1, the softest, to No. 4, the hardest, and most students use a No. 2. Pencils with letters use H for hard and B for black, so a 2B pencil is softer and darker than an HB.

Pencil makers push the graphite and clay mix through a small opening called a die to make long, thin rods. The rods are cut to length and baked hard in a kiln. Meanwhile, slats of incense-cedar get grooves cut along them. The rods lie in the grooves, a second grooved board is glued on top, and the wood is cut into pencils. Cedar is a good choice because it holds its shape when the heat or damp changes.

A pencil does not have to be made from a slat of wood. The Wopex pencil's body is about 70 percent wood and 30 percent plastic, and it is made by pushing the two out together with the graphite core. Whichever way it is made, a pencil can write for a very long time. A mathematician worked out that the graphite in just one pencil could draw a line over 700 miles long.

Words to know

• graphite: soft carbon form
• patent: inventor's license
• kiln: baking oven
• die: shaping hole