Why does a pencil write?

What is happening?

Graphite is a form of carbon built from stacked sheets of carbon atoms. The bonding within a sheet is strong, while the interactions between sheets are much weaker.

As the pencil moves across paper, tiny graphite-rich particles are rubbed from the core and become caught by the paper surface. Those particles make the visible line.

What changes it?

Changing the graphite-to-clay balance changes the pencil. More graphite generally gives a darker, softer mark; more clay produces a harder core and a lighter line.