How does the presence of a control group in an experiment allow you to test the null hypothesis?

I understand the concept behind the null hypothesis, but how is the control group related? Thanks|||So, say you want to see if some drug makes people use weight. You have two groups, one taking the drug another group taking a placebo, but you assume you have enough people in each group that other variables, besides taking or not taking the drug, are negligible.





You're null hypothesis is that there will be no difference in weight loss between the two groups.





Then, after some period of time, you weight the two groups, and see if there is a difference between them. If there is, then your null hypothesis is false and you can actually claim that the drug does effect how people lose weight.





Without the control group (those taking placebo) you have no basis for comparison. Even if the test group did lose weight, you can't say it was related to their taking the drug. Maybe they stopped eating junk food or there was some other factor that influenced their weight loss/gain.