A suitable control for an experiment is one that does not receive the independent variable. It is an experiment with all of the same constant variables applied, except it does not receive the independent variable. The result of this control is compared to the tested variable to determine if the effect of the independent variable had an effect. An example of this type of experiment is to use incandescent light placed at different distances from the plant to determine if light intensity had an effect on the rate of photosynthesis measured by the production of gas (oxygen). The test would consist of ten tubes and each one would have a chloroplast from a plant that would be placed in a tube along with distilled water. Tube 1 would be the control. It is wrapped in aluminum foil so it is not exposed to light. The tubes are placed in front of a light source, at different distances, and for different amounts of time each. The control is treated differently because, each time it is placed in front of the light source, it is covered by foil and the foil is placed back on afterwards.