imagine that we have two rabbits which we call Subject 1 (S1) and Subject 2 (S2). At S1 we feed it with grass, and at S2 we give it meat for a previously established time. After this process it is pertinent to ask the students: what will happen to S1 and S2? In what way could the changes produced by the feeding form of S1 and S2 be identified externally?