Chlorine and fluorine are some of the most dangerous, most reactive elemental gases on the planet.   But, of course, elements act very differently depending on what sort of compound they’re in.   I mean, add sodium to chlorine and you get table salt.   Add sodium to fluorine and you get… something that’ll strengthen your teeth?   Yeah -- that stuff your dentist calls fluoride is really sodium fluoride, or maybe another similar compound, like stannous fluoride or sodium monofluorophosphate.    What these substances have in common is that they contain fluoride ions.   But for a long time, nobody knew why fluoride helped prevent tooth decay, because like so many other great discoveries, it happened entirely by accident.   It all started in 1901, when a dentist named Frederick McKay moved to Colorado Springs.    Being a dentist, McKay spent a lot of time looking at other people’s teeth, and to his alarm, many of the local teeth were spattered with brown stains -- even teeth that were otherwise in perfect health.   But, despite their nasty looking teeth, the locals seemed to have more of a resistance to tooth decay.   He’d never heard of anything like this, and neither had any of his dentist friends. And it took him and his colleagues thirty years, but eventually they figured it out: The people of Colorado Springs had gross-looking, but healthy teeth because their water was full of naturally occurring fluoride.    In time, other researchers discovered that as long as water’s fluoride concentration was kept below 1 part per million, it would protect the teeth against cavities without those nasty brown splotches.   Which is awesome, because I like my teeth the color they are now.   Based on this evidence, in 1945, Grand Rapids, Michigan, became the first city in the United States to intentionally fluoridate its water.    And for the next fifteen years, researchers kept track of the rates of tooth decay among the city’s children.   The number of cavities dropped by sixty percent, and soon everybody was fluoridating their drinking water and toothpaste.   But still… dentists weren’t sure why the fluoride worked.   But by 1962, they’d figured it out. It turns out that fluoride’s effectiveness has a lot to do with the chemical composition of tooth enamel, which is the hard outer layer of the tooth.   Enamel contains a compound called hydroxyapatite.    When bacteria on your teeth start digesting the sugars in your mouth, they can produce enough acid to actually dissolve this hydroxyapatite -- which then produces calcium and phosphate ions.   Fluoride works by combining with these ions, producing a different compound, fluorapatite, which is actually incorporated back into the tooth as re-formed enamel.    And since it takes a higher concentration of acid to dissolve fluorapatite, this new enamel is much more resistant to decay.   So fluorine might be super dangerous, but fluoride is the best way to protect your teeth.   Thanks for asking, and thanks especially to all of our supporters on Patreon where if you support us at four dollars or more per month you can submit your questions to be answered right here on SciShow Quick Questions. And if you want to keep getting smarter with us, don’t forget to go to youtube.com/scishow, and subscribe.