A Century Old Discovery
Fluoride is the ion form of the element Fluorine. It occurs naturally all over the world. Almost 100 years ago, a Colorado dentist "discovered" that fluoride can make teeth stronger. His young patients had been raised in an area of Colorado that had a high concentration of fluoride in the water, naturally. Working with other scientists, he determined the amount of fluoride that is best to make people's teeth strong and beautiful. This led to dentists and scientists (same thing, mostly) to propose that we add the right amount of fluoride to the water people drink so that their teeth can be more resistent to tooth decay. In 1945, Newburgh NY and Grand Rapids MI became the first municipalities to add fluoride to their water supply. Many have followed, including Batavia.
Cold War Controversy
As most people know, the use of fluoride in water supplies became controversial because some people (who were not chemists or biologists or trained at all in the relevant fields) reported that Fluoride is a poison and said that fluoridating water was a plot by foreign communists to make us unhealthy. This happened at a time when America was very paranoid about the Soviet Union and communism. It was way too easy for many people to believe the hype.
Most of the controversy is over now because decades have passed and there has been no ill effects linked to water fluoridation. So even if the early scientists had been wrong about the safety of fluoride, we would have known by now because any side effects would have been seen. You can still find some very assertive conspiracy theorists on the internet who will try to convince you that you'll DIE if you drink fluoridated water, but then you can find every opinion somewhere on the internet. When faced with an internet claim that may seem reasonable, try to find out who disagrees with it, and why.
How It Works
Fluoride makes our teeth stronger by substituting itself into the crystal structure of the tooth. If there is a little fluoride in the body when the teeth are forming, the resulting crystals in the tooth are stonger. They become better able to resist the dissolving action of acids in our foods and are produced by decay bacteria. The surface of the teeth can also repair itself with fluoride - that's why we put fluoride into toothpaste and mouthwash.
Mother Nature Knows Best
Fluoride is a naturally occuring mineral that is totally safe at the small concentrations needed to maintain tooth strength. But you have to add the right amount. Too much fluoride in the water can make the teeth form with brown stains. They are still strong, but do not look right. The correct concentration of fluoride is about 1 part per million, which is 1 milligram in each liter of water. I find it facinating that fluoride in the oceans was at 1ppm millions of years ago, when bones and teeth first evolved. Mother nature intended us to have this small amount of natural fluoride in our teeth, but we left the oceans, walked up on dry land, and lost the benefits of fluoride.