Now that you have your scientific question, you must research more about your topic and narrow down your topic. For example if you want to know what makes the best paper airplane (not a good testable question) you need to determine one factor that might improve the distance that it flies. Decide if you want to test the design, weight, type of paper, how you throw it, or where you throw it. The factors that affect the results of an experiment are called variables.

There are 3 types of variables:

Independent Variable: (IV) “I Change” The one part of an experiment that is changed by the scientist. The IV is also called the manipulated variable because this is the variable that can affect the results.

Dependent Variable: (DV) “Results” The part of an experiment that may be affect as a result of changing the IV. It is what you measure to obtain your results. The DV is typically a number.

Constant Variable: (CV) “Stays the Same” The parts of the experiment that are kept the same throughout the experiment. The constant variables must stay the same so the results are accurate. For example you can not test the weight of the paper airplane and the design at the same time.

Here is an example of the IV, DV, and CV for the paper airplane experiment.
Question: How does the weight of a paper airplane affect the distance it flies?
IV: weight of the paper airplane
DV: distance the plane flies in meters
CV: the design of the plane, where you throw the plane, and how you throw the plane.