Klean Kanteens and denture tablets

We stood in the store looking at our phones, trying to figure out how to get the coffee stains out of our Klean Kanteens. Elaine had tried the official vinegar-and-baking-soda with no luck. I thought of denture tablets.

We bought three-minute store-brand tablets. Two tablets per Kanteen, warm water, an hour-long soak, and the stains were gone.

There is, of course, a body of lore concerning household uses for denture tablets. This use is the one I thought of and the one I can vouch for.