What is a constant in an experiment against which the results are compared?
In an experiment where a reaction is being tested, the use of a control gives a baseline on which the experimental results can be compared.
For instance, when a new drug is being tested and is in human trials, how do scientists know what the effects of the drug are? Let's take an example, let's say to combat pain in chronic pain patients. There is a well known reaction called the "placebo effect" which basically means that if someone thinks they are being cured, they will have a tendency to feel better. So how can we measure the effects of a drug when the participants in the study are already going to be feeling better?
You do it by dividing the study into 2 groups - one gets the trial medication and the other gets the placebo (and the participants don't know which group they are in). The results that scientists are looking for are the results in the trial group that go beyond what the control group experience (both good and bad).