And most people are immediately asking, "... huh? Why on earth would you do that?" I expect a few people are very interested, though.
I go through a lot of water, and I typically carry it around in a gallon jug. A typical milk jug doesn't work for long - they don't hold up. However, some places sell water in a heavier plastic jug. Mine, based on the faded label, is Nestle Pure Life. The heavier plastic (and more rounded edges) hold up great to the stresses of being a jug...
But the insides turn green over time. I drink straight from the jug mostly, and for reasons I probably don't want to understand, over time, I'll get a green buildup on the inside. Yuck. That's been my normal "end of life" for the jug in the past - I just haven't been able to find a good way to clean it out that works. Sloshing water around won't clean it, and I don't have any brushes that get the whole thing clean.
But I've found a solution! Ice cream rock salt!
I discovered that sea salt worked moderately OK for cleaning it a while back, but it tends to dissolve too quickly to be useful - it takes a bunch of times to clean it out, and it doesn't do a very good job. But ice cream rock salt (for an ice cream mixer)? That stuff works wonderfully! It's larger, heavier salt chunks that don't dissolve quickly - so I can put a couple cups of water in, toss some rock salt in, and shake it like a polaroid picture for a minute or two. Drain it, rinse it, and it's good as new!
My current jug is over a year old and still good as new inside!
Hopefully someone finds this useful!