What is a control variable?

i am doing a science fair project and i don't know what the control variable is. HELP ME!!!!!What is a control variable?
a control variable is when during an experiment you always have one beaker/petri dish that is never used in the experiment.





So, for example, if you did an experiment on baking soda and vinegar:








you have two beakers, both filled with vinegar. You add baking soda to one, and talk about what happened in your observations. The other beaker of vinegar is your control variable so you can compare between vinegar in its normal state versus when you add baking soda.





I hope this helps ! (:What is a control variable?
The control variable is the variable that isnt being tested on. (the placebo). The placebo is the fake medicine. then some people in the control group will have the placebo effect which is when people think they have the real medicine and they think it works but it really doesnt.
the variable that you use to compare your other tests with. For example if you are seeing how milk affects the carbonation of soda, Then your control would be just a regular soda with no mailk added.The control variable is never changed
blah
When one variable is tested at a time.