I learned of this hobby at a truck show awhile back. Truckers with some artistic talents, paint and design little rocks and leave them where other truckers will see them across the country. When discovered, the new caretaker of the rock will take it to another truck stop or rest area in another part of the country. I found one at the Kenly, NC Petro and I dropped it off in a prominent place at an Arizona rest area a few days later. They get around fast!
The Trucker Rocks 4Fun Facebook page can be found here. They have nearly 5000 members now and are still growing fast. Once you find a rock, you're supposed to post a picture of it and where you found it on the Facebook page. That way, everyone can see the movement of these rocks across the country. It's similar to the Where's George phenomenon that's been going on for years. Some rocks have the creator's initials and hometown on the back. Others have the Facebook page address on the back so finders can figure out what's going on and follow along.
Check out the Facebook page and notice the artistic detail on some of these rocks. Some are very impressive. Whoever came up with this hobby, had a pretty good idea. It's pretty neat to participate in a nationwide scavenger hunt of sorts.