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 investigating if solids dissolve in water. The thing that you are changing in this experiment (independent variable) is the solid (copper carbonate, salt, sugar, sand, copper sulfate). The thing that you will measure (dependent variable) is solubility. All other variables must be equal. This includes the amount of solid, the amount of water, the container and the temperature.