* The original "Pet Rock": The "Pet Rock" was a smooth, round rock marketed as a pet in 1975. It came in a cardboard box with air holes and instructions on how to "care" for your rock. This was all a joke, of course, but it became a huge sensation.
* Finding a similar rock: You could find a smooth, interesting-looking rock yourself and put it in a box.
* Making it "special": You could decorate the box, add instructions, and even give your rock a name.
Remember, a "pet rock" is really just a rock, but the fun is in the idea of it being a pet. You can have fun with it, but don't expect it to actually play fetch or give you cuddles!