In аn experiment оf the effect оf fertilizer оn tomаto plаnt growth, Rick decides to apply the fertilizer to only some of the plants and use the rest as controls to compare. He applies fertilizer to the 2 smallest plants. When he measures their growth at the end of the week, he finds the plants treated with fertilizer haven't grown as much as the untreated plants. What is the biggest mistake Rick made with his experiment?