The Essence of "Controlled Experiment"
One of the hallmark of science is the presence of a control set in an experiment. Sometimes this modality is known as "randomized trial and error". Many science teachers in elementary schools are cognizant of that and always try to "teach" students what does "control set" really mean. Alas, the nature of this "controlled experiment" technique is very subtle and not easy to be grasped. I myself made the mistake to gloss over it for decades. Until very recently I had an epiphany on the real essence of "controlled experiment". For example, if we want to know if water is a vital ingredient for photosynthesis, we build 2 sets of plants, one with water and another without (while keeping the remaining variables fixed for both plants). The corollary is that 1 of the 2 sets of plants must be able to grow. If it is the one with water hence it is self evidential that water is required, if it is the one without water then it is otherwise. It so