My office is situated right next to a Jewish cemetery. A coworker, noticing the yarmulke on my head, asked me why he sometimes sees stones placed on some of the gravestones. I told him that as far as I know the reason is to show respect for the deceased by showing that someone was there visiting the grave, but I'm wondering if there's a better or deeper reason. I've asked a few locals including the rabbi of my shul, but I have yet to find a good answer.