Artificial intelligence research on creative design has led to Structure-Behavior-Function (SBF) models that emphasize functions as abstractions for organizing understanding of physical systems. Empirical studies on understanding complex systems suggest that novice understanding is shallow, typically focusing on their visible structures and showing minimal understanding of their functions and invisible causal behaviors. In this paper, we describe an interactive learning environment called ACT (for Aquarium Construction Toolkit) in which middle-school students construct SBF models of complex systems as a vehicle for gaining a deeper understanding of how such systems work. We report on the use of ACT in middle-school science classrooms for stimulating, scaffolding, and supporting SBF thinking about aquarium systems as examples of complex systems. We present preliminary data indicating that SBF thinking, facilitated in part by the ACT tool, leads to enhanced understanding of the behaviors and functions of aquaria.