The purpose of this research is to search for the boundary between common sense and the lack of common sense. The same investigation was repeated four times for there years about four events ("Kiss", "Meal", "Chat", and "Portable telephone"). In each investigation, a similar questionnaire was used that described 15 situations about each of four events. Three groups of students were asked to fill in the extent to which they thought an individual situation to be strange. Dendrograms by the hierarchical cluster analysis showed constantly the figure with two cores about "Kiss" and "Meal". They also showed constantly the figure of one core about "Chat" and "Portable telephone". The former was judged to be a type with a clear boundary. As for the latter, some possibilities were discussed.
