Pencil
The important difference between pens and pencils is that the tip of a pencil is made of solid graphite (or other material) which is rubbed off onto the paper.  A pen has a tip, usually made of metal, with liquid ink coming out and onto the paper.  Writing with a pen can smudge when it is still wet.  Writing from a pencil can be erased, but writing from a pen usually cannot, unless it uses a special type of ink and eraser.