How does a pencil write?

A pencil writes because its core is graphite, a soft form of carbon that rubs off onto paper. Paper has tiny rough fibers, and they catch small bits of graphite as the pencil moves. More pressure leaves more graphite, making the mark darker.

Graphite is useful because it is arranged in flat layers that slide apart easily. An eraser works because those dark bits are resting on the paper fibers, not soaked in like ink, so rubber can lift many of them away.