Control and variable groups are used in order to test the hypothesis of an experiment. With the control group of an experiment, nothing is changed. Control groups are used to measure the changes that are produced by testing a hypothesis. While the control group stays the same, one thing is changed in the variable group in order to study the changes that are produced.
When testing a hypothesis, the independent variable is the part of the variable group manipulated by the experimenter in order to monitor the effects.
The dependent variable is the experimental response to the presence or absence of the independent variable in an experiment.
My turtle will grow faster if I give it one extra hour of lamplight every day.
A turtle that is the same size will be used as a control and will be given a normal amount of lamplight.