Rocks wear out the barrel in the middle, producing small holes which then leak the water and powder mix into the machine and all over the counter. If you are lucky, it will work once; the 2nd time, you have a mess halfway through the grinding/polishing process, with no way to finish, a worthless barrel and gears inside the machine that will grind themselves up.This was rec'd as a gift by my disappointed child. Note that it is sold under different brands, like EduScience (a ToysRUs label). Look CLOSELY at the barrel if you want to avoid this model; also, I think it is the only one with a clear plastic cover.