The present study aimed at investigating the process of acquiring knowledge in children. It was hypothesized that this was the internalizing process from an external act to an internal one. That was the following process: (i) the formation of an orientating basis,(ii) the formation of an act for the object,(iii) the transformation of the concrete form of the act into outer speech and (iv) the transformation of outer speech into inner speech. For this purpose, the acquisition process of the horizontality of water surface was examined by the method of the experimental education. First, the state of this acquisition was studied with one hundred 4-8 year-old children. Then two experiments, which followed Pre-test-Instruction-Post-test paradigm, were undertaken. In these experiments, unsuccessful Ss at a Pre-test were used. Experiment I: the acceleration of the acquisition caused by the experimental education was examined with eighty 5-8 year-old children. Experiment II: the difference of the effect of the acceleration of the acquisition caused by five different experimental educations was examined with. fifty 8 year-old children. The results were as follows: 1) Children's representation of water surface was. very difficult and was not perfectly attained by 8 year-old children. This result agreed with Piaget's findings. 2) The results of Exp. I clarified that water surface representation was easily influenced by the short term experimental education based on the above-mentioned hypothesis, And it was clarified that the instruction program, which taught water surface with reference to the table line on which bottles were placed, had some problems. The unscientific instruction program, in some cases, acted as check condition for the true acquisition. 3) The results of Exp. II showed that the instruction program based on the hypothesis, which taught water surface with reference to vertical line, was more effective than other instruction programs and that it had an effect not only on the acceleration of the acquisition but on its retention. So far as the present study is concerned, it seems to me that the acquisition of knowledge was accelerated by the instruction program, in which I intended Ss for internalizing the external act. But some problems still remain in this instruction program. Therefore, whether the process of acquiring knowledge is that of internalizing from external act to internal one or not, would be gradually clarified through the improvement of the instruction program, the electrophysiological investigation of inner speech and so on.