Painting rocks and leaving them for others to find is a popular pastime around here - kids especially enjoy it. The idea is that if you find one, you should enjoy it and re-hide it (not too well) so someone else can find it and enjoy it too. If you want to keep it, you're supposed to replace it with one of your own making. They are often inscribed with the location of origin followed by the word "Rocks" and the date. I've actually found some really awesome ones when I'm walking the dog but I always re-hide them since I don't paint any of my own to replace them with. Lots of kids will take pictures of them before they re-hide them and have a sort of competition to see who can find the most when they're out at a park or something. I've found rocks from several different states (and years) at the park I frequent, which has a campground associated with it.