My 2-year-old son is eating small black stones when he is playing outside. We were really shocked he considered those as "chips", and he tries to bite the stones and swallows them very often. Now we are not allowing him to play outside because we were really scared whether it will hurt his stomach.

Could any one please let us know how to stop this habit? How can we make him understand a "stone" is different from what he is thinking, and he's not allowed to eat it?