I'm sorry ..
PENCIL : Whenever I made a mistake, you're always here to erase it. But as you make my mistake vanish, you lose a part of yourself. You get smaller everytime.
ERASER : But I don't mind. I was made to help you whenever you do something wrong. Even though, I know soon I'll be gone and you have to replace me with a new one. So please, I hate to see you sad.