What Is A Pencil Sharpener?
Well if a pencil is blunt then you sharpen it with a pencil sharpener.It is usally shaped like a rectangle with a visable blade and a hole you stick a pencil in.

                        How It Works?
When the pencil is rotated inside the sharpener it press against the blade and then sharpens the pencil by removing the outer layer and then exposing the lead.

That is how and why a pencil sharpener works.