Some experiments divide subjects into two groups, the experimental group and the control group. The difference is that the experimental group is exposed to something related to the experiment and the control group is not exposed to it. In other words, some procedure is performed on the experimental group – it is the group exposed to the independent (manipulated) variable being tested in which the changes will be observed and recorded. A control group is kept separate from the rest of the experiment so it won’t affect the results. This isolates the independent variable's effects on the experiment and can help rule out alternate explanations of what happens. Not all experiments need a control group, but they are useful where the experimental conditions are complicated and many variables are involved. Experiments that use control groups are called controlled experiments. You don't need one in this case. In what you are thinking about doing, the 3 staining substances would likely take longer to stain one person's teeth than another - so the number of sets of teeth tested will be a variable. It appears the time in seconds (minutes?) would be your dependent variable. How will you measure when a stain appears? Take a video? Have independent observers judge when they first see a stain? Seems this will need to be thought out. The independent or manipulated variable will be the type os stain. They will have to be administered in the same way. Good luck!