Why is graphite slippery?

Hint: Some elements exist in more than one form of allotropes, this property is known as Allotropy. There are several allotropes of carbon, diamond and graphite are the two most important allotropes of carbon.

Complete answer:
These two allotropes of carbon; diamond and graphite, have different properties. Diamond is said to be the hardest naturally occurring substance while Graphite is one of the softest known substances.
Now coming to our question;
Graphite has a giant covalent structure in which the carbon atom forms layers of hexagonal rings. It forms a layered and planar structure. The single layer of graphite is known as graphene. There are no covalent bonds in between these layers. The forces between these layers in graphite are weak. As a result of which the layers can slide over one another. This makes graphite slippery. Due to this property of graphite, it is widely used as a lubricant.
Graphite also has delocalised electrons which can move between the layers in graphite. As a result of this graphite conducts electricity.

Additional information: In diamond there is a rigid network of carbon atoms, held together by strong covalent bonds, thus making it very hard. It finds application in cutting tools. Diamond doesn’t conduct electricity as there are no free electrons. Diamond is transparent while graphite is opaque.

Note:
We have seen that graphite is slippery and soft, contrary to that diamond is hard. Apart from these two allotropes of carbon there are many other allotropes like; Fullerenes, glassy carbon, amorphous carbon and others.