Pencil instead of Pen

Why do we use pencil instead of pen?
Who is sitting in the den.
He is all alone on the table right there
And is afraid to stand in a chair

If he had legs, he'd come up to you
And ask if you could use him too.

But he don't have legs, so he can't walk
And he doens't have a mouth, so he can't talk

But if he gets up, RUN.