There has been a body of emerging research describing students' understanding of complex systems. This research has primarily studied students understanding of complex phenomena in science. However, complex phenomena are also pervasive in everyday life. Children observe and participate in them daily. How do they reason about such ordinary complex phenomena? In this study, we investigate students' reasoning about everyday complex phenomena. We report on interviews and a classroom participatory simulation with ten sixth-grade students about ordinary events that could be construed as emergent, such as social situations in which the social pattern emerges from the participating students' individual actions. We have observed a widespread student-initiated strategy for making sense of complex phenomena. We call this strategy "mid level construction," the formation of small groups of individuals. Students form these mid-level groups either by aggregating individuals or by subdividing the whole group. We describe and characterize this mid-level strategy and relate it to the students' expressed understanding of "complex systems" principles. The results are discussed with respect to (a) students' strengths in understanding everyday complex social systems; (b) the utility of mid-level groups in forming an understanding of complex systems; (c) agent-based and aggregate forms of reasoning about complex systems. (Contains 3 tables, 4 figures, and 5 footnotes.)