The Pen That Sells Filters

Somewhere, right now, a salesman is dipping a little pen into a glass of someone’s tap water, watching a number climb, and watching their face fall. It’s the most reliable sales close in the water business — and it’s a magic trick. The number is real; what he tells you it means is not. Play it through below and watch the con from the inside.

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The short version, if you skipped to the end: a TDS meter measures dissolved solids, most of which are harmless minerals. It tells you nothing about the contaminants that can actually hurt you — lead, arsenic, PFAS, nitrate — because those are dangerous at concentrations far too small to move the needle. A high