You may have heard that a pencil is filled with lead. It is not. The black core is graphite mixed with clay. Long ago people thought graphite was a kind of lead, so they called it black lead. The name stuck, which is why we still say pencil lead today, even though the metal was never inside.

Graphite is one of the forms of pure carbon, and diamond is another. In diamond, each carbon atom is locked tightly to four others. In graphite, the atoms lie in flat sheets that slide over each other. Both are made of the same kind of atom, but graphite's sheets slide apart so easily that a pencil leaves a line when you drag it across paper.

The word graphite was invented in the 1780s, from a Greek word meaning to write. The word pencil comes from a Latin word for a small brush, and for a long time pencil meant a paintbrush. Before wood came into it, people wrote with graphite sticks, then wrapped them in string or sheepskin to hold them.

In the 1790s, a French engineer named Nicolas-Jacques Conté mixed graphite with clay and baked it in a kiln. That mix lets a maker choose how hard the pencil will be. More clay makes a harder pencil. More graphite makes a softer pencil with a darker line.

Pencil makers cut thin boards of wood and cut grooves along them. They lay graphite sticks in the grooves, glue a second board on top, and cut the wood sandwich into pencils. Many are made from cedar, which does not warp when the weather changes. Many are six-sided, because a round pencil can roll off a desk, and flat sides help it stay put.

Hardness is printed on the pencil. Numbers run from No. 1, the softest, to No. 4, the hardest, and most students in the United States use No. 2. Letters work too. H is for hard and B is for black, and HB sits in the middle. A 2B pencil is softer and darker than an HB.

Not every pencil has a slat, a thin board of wood. A pencil called Wopex, made by Staedtler, has a body of wood and plastic pushed out together, instead of a board of cedar. And the line any pencil draws is astonishing: a mathematician worked out that the graphite in one pencil could draw a line over 700 miles long. The line a soft 2B pencil leaves is only about 143 carbon atoms thick.

Words to know

• graphite: soft carbon form
• carbon: a common element
• kiln: baking oven
• slat: thin board