Use: to clean hard-to-clean bottles and line a soap dish.
To clean a narrow-necked bottle, pour some marbles, ball bearings, or small pebbles into the bottle and swish them around with soap and water. The marbles/pebbles will do the scrubbing that your hand can't. I first thought of this when I saw an ad for little stainless steel ball bearings that were designed specifically for this purpose. Why buy something specifically for one purpose only if you can use something you already have around and use somewhere else?
For example, I made a soap dish out of a small shallow tray that I filled with loose, rounded pebbles. It works great as a soap dish and looks nice. When I have a bottle to clean out or when it is time to clean out the soap dish, I just swish the pebbles around in the bottom of the bottle with some hot water. Everything comes out squeaky clean, and I've killed two birds with one stone (so to speak).