In an investigation there should only be one variable that is tested. This is called the independent variable.
Changing the independent variable causes a response in the dependent variable.
All other variables must be kept constant (controlled variables).
For example, when heating seeds. The thing that you are changing in this experiment (independent variable) is heat treatment. You will also have the same number of seeds that have not been heated. This is your control group. The thing that you will measure (dependent variable) is the germination of the seeds. All other variables must be equal. This includes the pot and soil that you use, heat and light and the amount of water (controlled variables).