Chemical reactions and pollutants
Scientists are interested in what happens, and what may change how these things happen. Something that happens is called an 'outcome'. Things that may change the way this happens are called 'factors'.
The quality of the air on a street is an example of an outcome. Factors that may affect this outcome include the number and type of vehicles using the street, whether there are factories along the street, the strength and direction of the wind, and so on.
If a factor is changed this may lead to a change in the outcome.
Some factors may increase the chances of an outcome but not inevitably lead to it. For example, smoking cigarettes increases the chances of a person getting lung cancer, but some people who smoke never get lung cancer.
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