In order to clarify the stages of explanation by the elementary school pupils from the first to the third grade, the following investigation was carried out.
Problem: The investigation asks if pupil's explanations differ from the first to the third grade.
Method: The sample comprised thirty pupils. Ten pupils were chosen in every grade by random sampling. They were told to arrange five pictures, which indicated the orders of changes of candle length, and their explanations of their seriation were analyzed.
Results: The results of this investigation are summarized as follows;
1) Pupils in every grade could arrange five pictures serially from the longest to the shortest candle.
2) Both the second and the third grade pupils could explain the seriation by succession law
