I had a transparent reusable water bottle I used to bring to my training hall, but I stopped using it because these gross black dots were forming on the inside near the bottom and they were difficult to scrub out, even with a bristle brush.
The developers of the Quartz Bottle know this problem well, so they designed a self-cleaning bottle:
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This is a brilliant idea, and I'm sad to have missed the IndieGogo campaign. The Quartz was 4,580% funded and should be rolling out in June of 2018. Retail units will reportedly be available for $99, and I'll be one of the first in line.