Explanation:

Pencil lead is formed of graphite. Graphite is an allotropic form of carbon in which each carbon atom is joined to three others, forming layers:

These layers are put together by weak van der Waals forces which enable the layers to slide over each other, making graphite soft and slippery. So graphite is used as pencil ‘lead’. As the pencil moves across the paper, layers of graphite rub off leaving the dark marks on paper.