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).
In this investigation the independent variable is the surface of the cans (shiny or dull). The dependent variable will be the rate of temperature change. All other factors should be controlled, these include the amount of radiation received the length of time in the sunlight, the thermometers and the cans themselves.