Control variable

Control variables are variables that stay the same in an experiment. They help scientists better understand the results of the experiment. For example, if you are doing an experiment to find out how different kinds of soil affect growth of plants, the control variable would be the amount of water, because that should stay the same no matter what type of soil you use for your experiment. That way, you can see how different types of soils affect the growth of plants, without worrying about the water having any effect on the results.