How do you figure out the variables in a science project?

The variable that you change is the independent variable(which you change). This could be the amount of light, fertilizer or salt that you give to a plant to observe how it affects its growth. What you measure is the dependent variable(the variables that change due to the change in independent variable) eg mass of the plant each day or week, number of leaves or height. All other variables are called the control variables(variables that are constant throughout the experiment). These make the experiment a "fair test". In the above experiment if you were to vary the amount of salt in the soil then each plant must be given the same amount of light, fertilizer, water etc.