Anyone with hard water in their community can relate to the problem of some pretty impregnable stains. Cool Tools has a nifty solution for this:
I used to scrub fruitlessly at toilet bowl rings until one day I tried pouring in some white vinegar (maybe a cup or two) and leaving it overnight. Everything came off in about a minute after that.
The tip goes on to add that if you're in a hurry, you can leave the vinegar in there for as little as twenty minutes; I've gotten this to work on showers and sinks as well.