After you have chosen your question and learned more about it, the next step is to come up with a possible answer. This is called "making a hypothesis". A hypothesis should explain how two different things are connected. For example, in the question above, we asked "Do sunscreen lotions with a higher SPF rating provide better protection?" The two things that are related are (1) the SPF rating of sunscreen lotion, and (2) the level of protection. So, the hypothesis could be "Sunscreen lotions with a higher SPF rating might block more UV radiation from the sun and give better protection."

It's important to have a hypothesis that can be tested and measured in a science project. One way to make sure your hypothesis can be measured is to identify the variables in your project. These are the things that can change or be compared.

The Independent variable

The independent variable is the thing that you change on purpose to see how it affects something else. This is called the dependent variable. In our example, the SPF rating of the sunscreen lotion would be the independent variable that we change by using different SPF ratings.

The Dependent variable

The dependent variable changes because of the independent variable. In our example, the level of protection would be the dependent variable, because it would change based on the SPF rating of the sunscreen lotion. We would then ask ourselves "How can we measure the level of protection?" Our research might tell us that we can use a UV meter to measure the amount of UV radiation.

The Control variables

The control variables are the things that stay the same. In our example, the independent variable is the SPF rating and the dependent variable is the level of UV radiation. Some of the control variables might be the brand of sunscreen lotion (using the same brand but different SPF ratings), the amount of sunscreen lotion used for each experiment, the type of materials and the type of UV meter used for each experiment. In a control, all the variables are the same as in the original experiment except for the independent variable. The variables that are the same in both the original experiment and the control are called control variables.