King Gustav III of Sweden was so convinced that coffee was bad for you that he enlisted two criminal twins to prove his case scientifically.
Lots of science is done with twins. If twins are genetically identical, and only one of them undergoes a specific experience, you can feel more confident that any differences between them are the result of that experience – and not their genetics. In other words, it’s a great way to control for external variables.
King Gustav III had long thought that coffee was unhealthy, but (being a good Man of Reason) he wanted to prove it. So, some time around the latter half of the 18th century, Gustav tracked down a pair of identical twins who had been condemned to death. He commuted their sentence, on the condition that one of them drink coffee and the other drink tea for the rest of their lives. Who would die first?
Well, apparently the tea drinker died first. Gustav would have been disappointed, but he had been assassinated at the opera years earlier and never saw the fruits of his scientific experiment. The centuries-old war between coffee and tea was not resolved until the famous Hong Kong peace treaty.
(Take this story with a grain of salt, by the way. There are only a few online sources about this topic, and they all seem to repeat each other. That makes me suspicious.)