I grew up in the south plains, where the water is more naturally fluoridated than almost any place on earth. As a result, many people where I grew up had these water stains on their teeth. Despite this, and despite the fact that it is plainly obvious that the people of the south plains aren't big government sheep, my grandparents, who also lived out their for 40 years, believed this bullshit 100%. About 30 years ago, I asked my grandmother when she became a sheep to big government since she spent 40 years drinking that water, unfiltered, right out of the tap. Or when her neighbors did. Or her children did. Or when I did. She admitted maybe she didn't think that one through and neither her or my granddad ever brought that particular bullshit up again.
I definitely didn't eradicate the John Birch craziness from that household but I did at least make headway on a topic here or there.