Understanding software structured as separate programs that communicate via message passing requires understanding how the various programs interact with each other, as well as understanding the individual programs in isolation. An interaction network display shows in a visual way message passing within and between programs. We have implemented an interaction network monitor for the MINIX operating system, for use in showing students how the system programs and processes of MINIX interact to provide operating system services. We give three examples to demonstrate the insights into message-based systems that can be gained from studying interaction network displays.