Complex systems are commonly found in natural and physical science. Understanding such systems is often difficult because they may be viewed from multiple perspectives and their analysis may conflict with or extend beyond the range of everyday experience. There are many complex structural, behavioral, and functional (SBF) relationships to understand as well. Design activities, which allow exploration of the way a system works and which eventually require deep understanding of that system for success, can be an excellent way to help children acquire a deeper, more systemic understanding of such complex domains. We report on a design experiment in which sixth grade children learned about the human respiratory system by designing and building artificial lungs. Students were interviewed pre- and postinstruction. Results of these interviews were analyzed using an SBF model for describing their understanding of the respiratory system. We consider the results in light of the children's actual activity and discuss some of the lessons learned.