Next, John performed an experiment on the effects of caffeine on body weight in mice. He had 2 groups each consisting of 2 mice. He weighed all the mice prior to the beginning of the experiment. For the next 120 days, one group of mice received a caffeine pill daily, while the other group did not. The only difference in the treatment and feeding of the mice was the caffeine supplementation. After the 120 days, he weighed all the mice again and meticulously recorded his results. John’s experiement has some flaws. What will be the most serious flaw of his experimental design?