Nine times out of ten a lab is most likely to start this behavior. Nobody really knows why, but some say that a lab wants something in its mouth no matter what it is. Rocks are the easiest and the most available.
One person we know of broke the rock problem by scattering a large amount of toys around the yard. Gave the dog something more fun to chew on.
If you see it packing another rock, make him drop it but reward him by offering a toy to play with instead.
