Coz.....its a graphite lead...and glass doesn't absorb it

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Pencil contain graphite with in it which composes of soft and loosely packed and paper too contains roughness due to it's irregularities, so there will be a space between molecules in paper. Such that they will separate from each other due to the frictional force appiled betweeen pencil and paper which we cant see in the case of graphite due to it doesnt absorb graphite since the molecules in it are tightly packed without having irregularities.
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