Reported is a method of investigating thought processes of an 11-year-old girl concerning physical phenomena such as those underlying the operations of scientific apparatus--platform balance, the spring balance, the magnet, and the pendulum. It was discovered during a period of interviews that the subject's thought processes developed in such a way that her conceptions of different phenomena became more interrelated, and the interrelationships became more dominant. The concept of "deep structure field" (d.s.f.) representing the basic processes that controlled the subject's conceptions of the phenomena, and thus her behavior, provided the theoretical orientation of the study. An analytical system based on the relationships between the properties of media, objects and the actions which were associated with them and comprised of cognitive relational categories (c.r.c.'s) was developed to distinguish the deep structure fields that controlled the subject. The underlying d.s.f.'s were identified in terms of the dynamic regularities in c.r.c.'s. A systematic search for regularities among the c.r.c.'s and their underlying d.s.f.'s and sub-d.s.f.'s provided support for the basic structural pattern with some sharpening of details. (Author/EB)