Now, if you are a scientist, obviously you will collect observations that support your explanation, but others are not likely to be persuaded just by a list of confirmations. Like proving RealClean detergent washes clothes best, it's easy to find—or concoct— circumstances that favor your view, which is why you have to bend over backward in setting up your test so that it is fair. So you set the temperature on both washing machines to be the same, you use the same volume of water, you use the recommended amount of detergent, and so forth. In the guppy case, you want to hold constant the amount of food in high-predation environments and low-predation environments, and so on. If you are wrong about the ability of RealClean to get the stains out, there won't be any difference between the two loads of clothes, because you have controlled or held constant all the other factors that might explain why one load of clothes emerged with fewer stains. You will have disproved your hypothesis about the allegedly superior stain-cleaning qualities of RealClean. You are conducting a fair test of your hypothesis if you set up the test so that everything that might give your hypothesis an advantage has been excluded. If you don't, another scientist will very quickly point out your error, so it's better to do it yourself and save yourself the embarrassment!

What makes science challenging—and sometimes the most difficult part of a scientific investigation—is coming up with a testable statement. Is the African AIDS epidemic the result of tainted oral polio vaccine (OPV) administered to Congolese in the 1950s? Chimpanzees carry simian immunodeficiency virus, which researchers believe is the source of the AIDS-causing virus HIV (human immunodeficiency virus). Poliovirus is grown on chimp kidney culture or monkey kidney culture. Was a batch of OPV grown on kidneys from chimps infected with simian immunodeficiency virus the source of African AIDS? If chimpanzee DNA could be found in the fifty-year-old vaccine, that would strongly support the hypothesis. If careful analysis did not find chimpanzee DNA, that would fail to support the hypothesis, and you would have less confidence in it. Such a test was conducted, and after very careful analysis, no chimp DNA was found in samples of the old vaccine. Instead, macaque monkey DNA was found (Poinar, Kuch, and Paabo 2001).

The study by Poinar and colleagues did not disprove the hypothesis that African AIDS was caused by tainted OPV (perhaps some unknown batch of OPV is the culprit), but it is strong evidence against it. Again, as in most science, we are dealing with probabilities: if all four batches of OPV sent to Africa in the 1950s were prepared in the same manner, at the same time, and in the same laboratory, what is the probability that one would be completely free of chimp DNA and one or more other samples would be tainted? Low, presumably, but because the probability is not 0 percent, we cannot say for certain that the oPV-AIDS link is out of the question. However, we have research from other laboratories on other samples, and they also were unable to find any chimpanzee genes in the vaccine (Weiss 2001). Part of science is to repeat tests of the hypothesis, and when such repeated tests confirm the conclusions of early tests, it greatly increases confidence in the answers. Because the positive evidence for this hypothesis for the origin of AIDS was thin to begin with, few people now are taking the hypothesis seriously. Both disproof of hypotheses and failure to confirm are critical means by which we eliminate explanations and therefore increase our understanding of the natural world.

Now, you might notice that although I have not defined them, I already have used two scientific terms in this discussion: theory and hypothesis. You may already know what these terms mean—probably everyone has heard that evolution is "just a theory," and many times you have probably said to someone with whom you disagree, "Well, that's just a hypothesis." You might be surprised to hear that scientists don't use these terms in these ways.

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