Introduction to Biology: Scientific method, experimentation.
In the scientific method, experiments are used to prove or discard a hypothesis. They are done in a controlled environment so that more information can be obtained from them.
The variables are things that can change. For example, the growth of a plant (how tall it gets or how many leaves it has) is a variable, other variables are how much water or sunlight it receives.
Variables can be dependent or independent. In this example, the growth of the plant is the dependent variable, because it is affected by the independent variables, in this case, how much water and sunlight the plant had.
In an experiment, we modify an independent variable to see how the dependent variable is affected, but to know this, we need to have a control group, which is the baseline for the other groups.
For example, if we want to know how the quantity of water affects plant growth, we have two groups of plants of the same age, in the same type of soil and pot, in the same place, and we change the quantity of water of group 2, then group 1 would be the control group.