You find interesting little rocks; ones with some colors and different textures. You want rocks that are about the size of a marble, maybe a little bigger. Then you put them in the tumbler with some grit and water, and turn it on. It's tumbles the rocks, and the grit takes the edges off and starts to smooth it out. After a while, you turn it off and remove the rocks and grit. Then, you put them back in with some more water and a finer grit, and let it run again. You do this a few times, each time using a finer grit. You end up with a totally smooth, polished rock with an almost glass-like surface. It really brings out the colors and details. You can build a little collection of cool rocks with various colors and detail.