If you read newspaper articles about science, you'll see the words "theory" and "hypothesis" tossed around as if they both mean "unproven idea." It's not so. A scientific hypothesis is a question or prediction about something. If the prediction holds up after scientific tests, then it earns standing as a theory.
A hypothesis is a guess about something when you don't know the answer. A lot of scientific hypotheses are written as "if...then" statements. For example, if you think a particular chemical is toxic, the hypothesis might be "If test animals are exposed to this chemical, then they will develop cancer."
A hypothesis for a science project, or a professional research project, has to be testable. If you can't think of a way to prove your hypothesis true or false, it's not usable. A hypothesis saying "If you talk to plants four hours a day, they'll grow faster" is testable, for instance: you grow two sets of plants, and talk to one and not to the other. If they live in otherwise identical conditions, their growth rates should show whether talking has any effect.
You should come up with your hypothesis before you design your experiment. If you're testing a hypothesis about plant growth, for instance, you want more than one plant in each group. That reduces the distortion effect of say, a single plant being weak and not growing as much as normal. The simplest experiment is one where the result is either/or. If the plants you talk to grow more, that's evidence your hypothesis is right; if they don't grow more, the hypothesis fails.
No Sure Things
A hypothesis that survives rigorous, repeated tests under tightly controlled conditions becomes a theory. While the word "theory" as used in everyday life means something that isn't proven, in science a theory has solid evidence behind it. It's still only a theory because it's always possible someone down the road will discover evidence against it. That's why even established scientific facts such as evolution and relativity are still referred to as theories. Nothing is ever 100 percent sure in science.
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