The pencil mark is made up of graphite particles. It is easy to erase when it is freshly, made because the graphite has not had time to penetrate into the tiny hollows of the paper.
The pencil mark also contains some wax. When the wax is fresh it seeps into the paper taking some particles of graphite along with it. When the wax hardens the graphite is fixed firmly onto the paper, and then it is not as easy to erase the mark as it was when it was freshly made.
Picture Credit : Google
