参考资料：
[1]https://en.wikipedia.org/wiki/Water_fluoridation_controversy
[2]https://zh.wikipedia.org/wiki/%E6%B0%9F%E5%8C%96%E7%89%A9
[3]https://www.nhs.uk/live-well/healthy-teeth-and-gums/how-to-keep-your-teeth-clean/
[4]https://www.zhihu.com/question/20093857
[5]https://www.smithsonianmag.com/science-nature/from-gunpowder-to-teeth-whitener-the-science-behind-historic-uses-of-urine-442390/
[6]https://baike.baidu.com/item/%E7%89%99%E8%8F%8C%E6%96%91/5093064
