Mom had this bottle filled with some old vinegar and after emptying it out there was still some gunk on the interior walls that would't come out. I heard about this method of using rice somewhere ( Martha?) and decided to try it out.
All you need to do is to pour in a couple of table spoons of uncooked rice in the bottle, add an inch of water and then shake vigorously for a few minutes.
And it works! I didn't expect it to work that well but it did. The rice scrubbed the inside clean. I've heard coarse salt works too. Will try that next time!