Sheets that slide off
Why Does a Pencil Leave a Mark?
Pencil lead is carbon in flat sheets that barely hold on to each other, so paper scrapes them off a layer at a time.
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Step 1 of 5
There is no lead in a pencil.
The name stuck from a 16th-century graphite find that people took for a kind of lead. What is actually in there is carbon and clay.
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Step 2 of 5
Graphite is carbon stacked in sheets.
Within a sheet every atom is tightly bonded to three neighbours. Between the sheets there is almost nothing holding on, so they slide apart easily.
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Step 3 of 5
Paper is rough enough to catch them.
Paper looks smooth but is a tangle of fibres. Dragging graphite across it snags sheet after sheet away and leaves them lying in the valleys.
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Step 4 of 5
Clay is what makes a pencil hard or soft.
More clay means the sheets are held together better, so fewer come off and the line is fainter. More graphite means a darker, smudgier line.
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Step 5 of 5
An eraser is stickier than the paper.
Rubber dragged over the mark grabs the loose graphite flakes more strongly than the paper fibres do, and carries them off in the crumbs.
A single pencil can draw a line around 50 km long, because each flake it leaves behind is only a few atoms thick.
The short version
A pencil writes because graphite is carbon in flat sheets held together weakly, and rough paper scrapes those sheets off as you drag it.
Try it yourself
Scribble a thick patch of pencil on paper and rub your finger over it. It feels slippery — you are sliding on sheets of carbon.
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