​It is important for an experiment to be a fair test. To ensure that your experiment is a fair test, you must change only one factor at a time while keeping all other conditions the same. Why? If you changed more than one variable it would be hard to figure out which change is causing what you observe.

 

The things that are changing in an experiment are called variables. A variable is any factor, trait, or condition that can exist in differing amounts or types. An experiment usually has three kinds of variables: independent, dependent, and controlled.

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Independent variable is the variable that changes - this is based off what is being tested. Usually, there is only one independent variable. The dependent variable is the variable that is being observed to see how it responds to the changes in the independent variable. Controlled variables are the variables that are kept constant throughout the experiment. There can be many controlled variables in an experiment.