You know how a really incompetent cook "doesn't know how to boil water"? Well, there is at least one place on the web that will teach you how to boil water. It has all kinds of interesting details about boiling water, including what temperature a simmer occurs at (185°F at sea level) and what things affect the boiling point. I found it when I needed to know the boiling temperature of water here, 5000 feet above sea level, so I would know how hot the jelly I was making should get to jell properly. It turns out that the boiling temperature of water drops 2°F for every 1000 feet of elevation gain, so water boils at 202°F here instead of 212°. That means that the syrup turns into jelly at 210° instead of 220°.
The jelly I was making is crabapple jelly, and it turned out really well. But after all the research, I couldn't get my thermometer to work the way it should, and the spoon-drop tricks in The Joy of Cooking never work for me, so I had to eyeball it. My jars came out nicely jelled anyway, at least once I switched to the right size pot; the large one I was using at first let the syrup cool too much around the edges, so the jelly came out very well set. Crabapples make a great jelly, not too sweet and very pretty. It's worth all the work (although I didn't neccesarily think so when I finished up last night).