 What is a variable?  A variable is anything that can affect the results in an experiment.

 How do variables in an experiment work?  In order to have a reliable experiment, you can only change ONE thing we are testing!  If we change more than one thing, we will not know why the experiment worked the way it did.

 We are testing two types of fertilizer (miracle grow and speedy grow) to see which plant grows the tallest.  We will use the same plant, the same amount of water, and the same amount of light.  The ONLY variable we change or that is different is the type of fertilizer!  Since we don’t change anything else, we can see the affect the fertilizer has on the plants. Type of plant Type of fertilizer Amount of water Amount of sunlight Height of plant FernMiracle Grow10 mL5 hours5.7 cm FernSpeedy Grow10 mL5 hours3.4 cm

 We are testing different fertilizers to see which one makes the plant grow the tallest.  The amount of water and sunlight are the same.  The type of fertilizer AND the type of plant are different.  Since there is more than one variable different, this experiment is not reliable or valid. We don’t know which variable (fertilizer or plant) affected the growth. Type of plant Type of fertilizer Amount of water Amount of sunlight Height of plant FernMiracle Grow10 mL5 hours5.7 cm Spider plantSpeedy Grow10 mL5 hours3.4 cm