What is the purpose of a variable/control in an experiment?

Help me answer please :D What is the purpose of a variable/control in an experiment?
to observe if the variable is responsible for the response


is the salt in salt water making the water salty? well taste distilled water, and then distilled water with salt... the salt in the distilled water is the variable. Its not perfect but you get the idea i hopeWhat is the purpose of a variable/control in an experiment?
A control is a fancy way of saying a reference point. In the scientific method, one is usually comparing the results of several runs of the experiment, often times changing one variable for each run. The control would be an initial run of the experiment without changing anything. The results are used as a reference point for comparing the results of the other runs to. With a control run, one can produce results that are expressed as %. For example, one could increase ingredient 'A' by 50% and the rate of reaction increased by 20%. A percent is always a ratio, the control is what is on the bottom of the fraction. In the above example, (mass of 'A' in experimental run) / (mass of 'A' in control run) * 100% = 150% and (rate of reaction in experimental run) / (rate of reaction in control run) * 100% = 125%