What is the purpose of the control group in a controlled experiment?

That is my question in biology homework. Help me out.|||To have a group of stable and unchanged variables to compare and analyze results to.|||You need to have a control group so you can compare the results of between the variables and the control group. For example, you are experimenting whether feeding more food to a fish would increase its growth rate. Your variable would be the extra food. And your control would be normal amount of food. Thus, you can definitely say that only food was the factor in the results.|||To allow the person doing the experiment to compare the results they get with normal results