The purpose of this study was to investigate the characteristics of reasoning in which $3^{rd}$ grade elementary school students form ideas, design experiments, and interpret the results to solve problems in small group argumentation. For this purpose, 12 3rd-grade students' small group argumentations including experiments were observed. The researchers analyzed students' pre- and post-open questionnaires, field notes, and video recordings of small group argumentation. The results of the research are as follows. First, in the initial opinion formation process, a hasty unification of opinions and a transformation of inquiry problem occurred. In the design and execution of experiments, verification experiments and unplanned and arbitrary experiments were performed. They also selectively noticed or accepted claims, evidence, interpretation, and criticism. They could distinguish between the condition and the cause, but they were confused by using inaccurate terms and tended to keep the initial opinions when interpreting the results and drawing conclusions.