Here's why:

* Independent Variable: The amount of vinegar and baking soda used (or perhaps the concentration of the vinegar) is the independent variable. This is what you are changing to see the effect.

* Dependent Variable: The size of the balloon is what you are measuring and observing. It *depends* on how much vinegar and baking soda are used.

The experiment is designed to see how changing the amount of vinegar and baking soda affects the size of the balloon, which is the dependent variable.