If you want to know why the debate over teaching evolution remains so contentious, consider the stickers some school boards have wanted to paste in high school biology textbooks. They label evolution a “theory, not a fact,” suggesting that an alternative explanation is possible. It's a clever strategy. Even people sympathetic to evolution often don't know how to respond to the assertion that evolution is “just a theory.” And the opposite claim―that evolution is a fact―can generate skepticism among those who don't like to be told what to think. But these stickers use the words “theory” and “fact” in a very misleading way. The biggest problem is that “theory” has two separate meanings. In common usage, “theory” means an idea or a hunch: That's not what “theory” means within science. When scientists speak of the theory of gravitation or evolutionary theory, they are talking about scientific concepts that have been so thoroughly tested that they are very unlikely to change. Theories are the results of decades or centuries of scientific effort. In science, a hunch or conjecture is called a hypothesis, not a theory. Copernicus’ idea was a hypothesis at first. But four centuries of observation and thinking have convinced us that heliocentrism is a theory, not just an idea. ＊ 太陽内部では４個の水素原子がヘリウムに変化する核融合で発熱しています。 It is compatible with observations of solar system that cannot be explained in other ways. “Theory” is doing double duty. So calling evolution a theory may seem to denigrate it in everyday terms, but in scientific terms that's high praise. “Fact,” on the other hand, is a word that makes many scientists uncomfortable. It implies that something is true beyond doubt. But the definition of a scientific statement is that its accuracy can be tested by comparing it to observations in the natural world; in other words, a scientific statement must be falsifiable. Accepting that something is true beyond doubt requires an act of faith. Researchers continue to investigate many interesting questions about evolution, such as how quickly or slowly it takes place. But studying whether evolution has occurred would be akin these days to studying whether the sun revolves around the Earth. No empirical evidence supports the hypothesis put forth by creationism―that God either created the world in its current form or directed the process of evolution. On the contrary, we would need foolish patience to believe that God created the appearance of evolution as an illusion to test our faith. If the proponents of creationism uncovered some facet of the biological world that contradicted the theory of evolution, some scientists would be intensely interested. Coming across an exception in science is like winning the lottery. Unexplained findings can offer a shortcut to scientific fame, as when Alexander Fleming noticed that mold was preventing the growth of staph bacteria in culture. ＊Fleming はアオカビがstaph bacteria（黄色ブドウ球菌）の増殖を妨げることを偶然発見して、そこから抗菌物質の生産に成功した。これはアオカビの属名Penicilliumにちなんで、ペニシリンと名づけられた。 But creationists have not come up with a single scientific observation that undercuts evolution. Biologists have demolished the few arguments that creationists have proposed. And at this point, creationist organizations are spending most of their money on public relations rather than research.